feat(pool-manager): add /healthz handler for k8s liveness/readiness probes (#56)

- pool-manager/main.go: serve HTTP on :3990 with /healthz that does a 2s-context
  pgxpool.Ping; returns 200 OK when Postgres is reachable, 503 otherwise.
  Runs alongside the existing signal-blocking main loop.
- pool-manager/Dockerfile: distroless static-debian12 image exposing :3990.
- pool-manager/.gitignore: skip built pool-manager binary and *.tar.

Refs marcus/engineering#56
Closes marcus/engineering#56

pool-manager/.gitignore

@@ -0,0 +1,2 @@
+pool-manager
+*.tar

pool-manager/Dockerfile

@@ -0,0 +1,8 @@
+FROM gcr.io/distroless/static-debian12:nonroot
+WORKDIR /app
+COPY pool-manager /app/pool-manager
+EXPOSE 3990
+ENV PORT=3990 \
+    HOST=0.0.0.0 \
+    POOL_CONFIG=/app/harness.yaml
+ENTRYPOINT ["./pool-manager"]

pool-manager/main.go

@@ -0,0 +1,59 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"scobleclaw.dev/pool-manager/poolq"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+func main() {
+	dsn := os.Getenv("PG_DSN")
+	if dsn == "" {
+		dsn = "host=/var/run/postgresql dbname=postgres sslmode=disable"
+	}
+
+	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
+	defer stop()
+
+	pool, err := pgxpool.New(ctx, dsn)
+	if err != nil {
+		log.Fatalf("pgxpool: %v", err)
+	}
+	defer pool.Close()
+
+	if err := poolq.EnsureSchema(ctx, pool); err != nil {
+		log.Fatalf("ensure schema: %v", err)
+	}
+
+	log.Println("pool-manager schema ensured, poolq ready")
+
+	mux := http.NewServeMux()
+	mux.HandleFunc("/healthz", func(w http.ResponseWriter, r *http.Request) {
+		pingCtx, cancel := context.WithTimeout(r.Context(), 2*time.Second)
+		defer cancel()
+		if err := pool.Ping(pingCtx); err != nil {
+			w.WriteHeader(http.StatusServiceUnavailable)
+			fmt.Fprintln(w, "pg unreachable:", err)
+			return
+		}
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprintln(w, "ok")
+	})
+	go func() {
+		log.Println("healthz server listening on :3990")
+		if err := http.ListenAndServe(":3990", mux); err != nil {
+			log.Fatalf("healthz server: %v", err)
+		}
+	}()
+
+	<-ctx.Done()
+}