feat: add SQLite data store with Lesson and LessonMedia models

- Add Microsoft.Data.Sqlite NuGet package to csproj
- Create Lesson model (Id, Title, Grade, Subject, LessonDate, Description, SourcePath, CompiledHtml, CreatedAt, UpdatedAt)
- Create LessonMedia model (LessonId, JellyfinItemId, SortOrder)
- Create LessonStore with SQLite-backed CRUD operations:
  - Constructor expands ~ in dbPath to home directory
  - Creates lessons and lesson_media tables on init
  - GetAllLessons(), GetLessonById(), GetLessonsByGrade()
  - UpsertLesson() with media association handling
  - DeleteLesson() with FK cascade
- Enables foreign key enforcement via PRAGMA
- Build passes with 0 warnings, 0 errors

Closes #4

plugin/Data/LessonStore.cs

@@ -0,0 +1,223 @@
+#nullable enable
+using System;
+using System.Collections.Generic;
+using System.IO;
+using Microsoft.Data.Sqlite;
+using JellyfinLessons.Data.Models;
+
+namespace JellyfinLessons.Data;
+
+/// <summary>
+/// SQLite-backed data store for lessons and their media associations.
+/// </summary>
+public class LessonStore
+{
+    private readonly string _connectionString;
+
+    /// <summary>
+    /// Creates the data store, expanding ~ in the dbPath to the user's home directory.
+    /// </summary>
+    public LessonStore(string dbPath)
+    {
+        if (dbPath.StartsWith("~/") || dbPath == "~")
+        {
+            var home = Environment.GetFolderPath(Environment.SpecialFolder.UserProfile);
+            dbPath = Path.Combine(home, dbPath.TrimStart('~', '/'));
+        }
+
+        _connectionString = $"Data Source={dbPath}";
+        CreateTables();
+    }
+
+    private void CreateTables()
+    {
+        using var connection = new SqliteConnection(_connectionString);
+        connection.Open();
+
+        using var command = connection.CreateCommand();
+        command.CommandText = @"
+            CREATE TABLE IF NOT EXISTS lessons (
+                id TEXT PRIMARY KEY,
+                title TEXT NOT NULL,
+                grade INTEGER NOT NULL,
+                subject TEXT NOT NULL,
+                lesson_date TEXT NOT NULL,
+                description TEXT,
+                source_path TEXT NOT NULL,
+                compiled_html TEXT,
+                created_at TEXT NOT NULL,
+                updated_at TEXT NOT NULL
+            );
+
+            CREATE TABLE IF NOT EXISTS lesson_media (
+                lesson_id TEXT NOT NULL,
+                jellyfin_item_id TEXT NOT NULL,
+                sort_order INTEGER NOT NULL DEFAULT 0,
+                PRIMARY KEY (lesson_id, jellyfin_item_id),
+                FOREIGN KEY (lesson_id) REFERENCES lessons(id) ON DELETE CASCADE
+            );
+        ";
+        command.ExecuteNonQuery();
+
+        using var pragmaCmd = connection.CreateCommand();
+        pragmaCmd.CommandText = "PRAGMA foreign_keys = ON;";
+        pragmaCmd.ExecuteNonQuery();
+    }
+
+    /// <summary>
+    /// Returns all lessons (without media).
+    /// </summary>
+    public List<Lesson> GetAllLessons()
+    {
+        var lessons = new List<Lesson>();
+        using var connection = new SqliteConnection(_connectionString);
+        connection.Open();
+        using var command = connection.CreateCommand();
+        command.CommandText = "SELECT * FROM lessons ORDER BY lesson_date DESC, title ASC";
+        using var reader = command.ExecuteReader();
+        while (reader.Read())
+            lessons.Add(MapLesson(reader));
+        return lessons;
+    }
+
+    /// <summary>
+    /// Returns a lesson by its id, or null if not found.
+    /// </summary>
+    public Lesson? GetLessonById(string id)
+    {
+        using var connection = new SqliteConnection(_connectionString);
+        connection.Open();
+        using var command = connection.CreateCommand();
+        command.CommandText = "SELECT * FROM lessons WHERE id = @id";
+        command.Parameters.AddWithValue("@id", id);
+        using var reader = command.ExecuteReader();
+        if (reader.Read())
+        {
+            var lesson = MapLesson(reader);
+            lesson.MediaItems = GetMediaForLesson(id);
+            return lesson;
+        }
+        return null;
+    }
+
+    /// <summary>
+    /// Returns all lessons for a given grade level.
+    /// </summary>
+    public List<Lesson> GetLessonsByGrade(int grade)
+    {
+        var lessons = new List<Lesson>();
+        using var connection = new SqliteConnection(_connectionString);
+        connection.Open();
+        using var command = connection.CreateCommand();
+        command.CommandText = "SELECT * FROM lessons WHERE grade = @grade ORDER BY lesson_date DESC, title ASC";
+        command.Parameters.AddWithValue("@grade", grade);
+        using var reader = command.ExecuteReader();
+        while (reader.Read())
+            lessons.Add(MapLesson(reader));
+        return lessons;
+    }
+
+    /// <summary>
+    /// Inserts or updates a lesson and its associated media entries.
+    /// </summary>
+    public void UpsertLesson(Lesson lesson, List<LessonMedia> media)
+    {
+        using var connection = new SqliteConnection(_connectionString);
+        connection.Open();
+        using var transaction = connection.BeginTransaction();
+
+        using var command = connection.CreateCommand();
+        command.CommandText = @"
+            INSERT INTO lessons (id, title, grade, subject, lesson_date, description, source_path, compiled_html, created_at, updated_at)
+            VALUES (@id, @title, @grade, @subject, @lessonDate, @description, @sourcePath, @compiledHtml, @createdAt, @updatedAt)
+            ON CONFLICT(id) DO UPDATE SET
+                title = excluded.title,
+                grade = excluded.grade,
+                subject = excluded.subject,
+                lesson_date = excluded.lesson_date,
+                description = excluded.description,
+                source_path = excluded.source_path,
+                compiled_html = excluded.compiled_html,
+                updated_at = excluded.updated_at;
+        ";
+        command.Parameters.AddWithValue("@id", lesson.Id);
+        command.Parameters.AddWithValue("@title", lesson.Title);
+        command.Parameters.AddWithValue("@grade", lesson.Grade);
+        command.Parameters.AddWithValue("@subject", lesson.Subject);
+        command.Parameters.AddWithValue("@lessonDate", lesson.LessonDate);
+        command.Parameters.AddWithValue("@description", (object?)lesson.Description ?? DBNull.Value);
+        command.Parameters.AddWithValue("@sourcePath", lesson.SourcePath);
+        command.Parameters.AddWithValue("@compiledHtml", (object?)lesson.CompiledHtml ?? DBNull.Value);
+        command.Parameters.AddWithValue("@createdAt", lesson.CreatedAt);
+        command.Parameters.AddWithValue("@updatedAt", lesson.UpdatedAt);
+        command.ExecuteNonQuery();
+
+        using var deleteMediaCmd = connection.CreateCommand();
+        deleteMediaCmd.CommandText = "DELETE FROM lesson_media WHERE lesson_id = @lessonId";
+        deleteMediaCmd.Parameters.AddWithValue("@lessonId", lesson.Id);
+        deleteMediaCmd.ExecuteNonQuery();
+
+        foreach (var item in media)
+        {
+            using var insertMediaCmd = connection.CreateCommand();
+            insertMediaCmd.CommandText = "INSERT INTO lesson_media (lesson_id, jellyfin_item_id, sort_order) VALUES (@lessonId, @jellyfinItemId, @sortOrder)";
+            insertMediaCmd.Parameters.AddWithValue("@lessonId", lesson.Id);
+            insertMediaCmd.Parameters.AddWithValue("@jellyfinItemId", item.JellyfinItemId);
+            insertMediaCmd.Parameters.AddWithValue("@sortOrder", item.SortOrder);
+            insertMediaCmd.ExecuteNonQuery();
+        }
+
+        transaction.Commit();
+    }
+
+    /// <summary>
+    /// Deletes a lesson and its associated media entries (cascaded by FK if enabled).
+    /// </summary>
+    public void DeleteLesson(string id)
+    {
+        using var connection = new SqliteConnection(_connectionString);
+        connection.Open();
+        using var command = connection.CreateCommand();
+        command.CommandText = "DELETE FROM lessons WHERE id = @id";
+        command.Parameters.AddWithValue("@id", id);
+        command.ExecuteNonQuery();
+    }
+
+    private List<LessonMedia> GetMediaForLesson(string lessonId)
+    {
+        var items = new List<LessonMedia>();
+        using var connection = new SqliteConnection(_connectionString);
+        connection.Open();
+        using var command = connection.CreateCommand();
+        command.CommandText = "SELECT * FROM lesson_media WHERE lesson_id = @lessonId ORDER BY sort_order ASC";
+        command.Parameters.AddWithValue("@lessonId", lessonId);
+        using var reader = command.ExecuteReader();
+        while (reader.Read())
+        {
+            items.Add(new LessonMedia
+            {
+                LessonId = reader.GetString(0),
+                JellyfinItemId = reader.GetString(1),
+                SortOrder = reader.GetInt32(2)
+            });
+        }
+        return items;
+    }
+
+    private static Lesson MapLesson(SqliteDataReader reader)
+    {
+        return new Lesson
+        {
+            Id = reader.GetString(0),
+            Title = reader.GetString(1),
+            Grade = reader.GetInt32(2),
+            Subject = reader.GetString(3),
+            LessonDate = reader.GetString(4),
+            Description = reader.IsDBNull(5) ? null : reader.GetString(5),
+            SourcePath = reader.GetString(6),
+            CompiledHtml = reader.IsDBNull(7) ? null : reader.GetString(7),
+            CreatedAt = reader.GetString(8),
+            UpdatedAt = reader.GetString(9)
+        };
+    }
+}

plugin/Data/Models/Lesson.cs

@@ -0,0 +1,65 @@
+#nullable enable
+using System.Collections.Generic;
+
+namespace JellyfinLessons.Data.Models;
+
+/// <summary>
+/// Represents a homeschool lesson plan.
+/// </summary>
+public class Lesson
+{
+    /// <summary>
+    /// Unique identifier (Guid).
+    /// </summary>
+    public string Id { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Lesson title.
+    /// </summary>
+    public string Title { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Target grade level.
+    /// </summary>
+    public int Grade { get; set; }
+
+    /// <summary>
+    /// Subject area (e.g. Math, Science, History).
+    /// </summary>
+    public string Subject { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Date the lesson covers (ISO 8601 format).
+    /// </summary>
+    public string LessonDate { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Optional description of the lesson.
+    /// </summary>
+    public string? Description { get; set; }
+
+    /// <summary>
+    /// Path to the source markdown or plan file.
+    /// </summary>
+    public string SourcePath { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Pre-compiled HTML rendering of the lesson.
+    /// </summary>
+    public string? CompiledHtml { get; set; }
+
+    /// <summary>
+    /// ISO 8601 creation timestamp.
+    /// </summary>
+    public string CreatedAt { get; set; } = string.Empty;
+
+    /// <summary>
+    /// ISO 8601 last-updated timestamp.
+    /// </summary>
+    public string UpdatedAt { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Associated media items (lazy-loaded).
+    /// </summary>
+    public List<LessonMedia> MediaItems { get; set; } = new();
+}

plugin/Data/Models/LessonMedia.cs

@@ -0,0 +1,22 @@
+namespace JellyfinLessons.Data.Models;
+
+/// <summary>
+/// Join model linking lessons to Jellyfin media items.
+/// </summary>
+public class LessonMedia
+{
+    /// <summary>
+    /// Foreign key to the lesson.
+    /// </summary>
+    public string LessonId { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Identifier of the Jellyfin media item.
+    /// </summary>
+    public string JellyfinItemId { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Display order within the lesson.
+    /// </summary>
+    public int SortOrder { get; set; }
+}

plugin/Jellyfin.Plugin.Lessons.csproj

@@ -12,5 +12,8 @@
     <PackageReference Include="Jellyfin.Model" Version="10.10.*">
       <ExcludeAssets>runtime</ExcludeAssets>
     </PackageReference>
+    <PackageReference Include="Microsoft.Data.Sqlite" Version="9.0.*">
+      <ExcludeAssets>runtime</ExcludeAssets>
+    </PackageReference>
   </ItemGroup>
 </Project>