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
deecc1dd7c