feat: add DB-backed scanner entrypoint and compute-only main hook

Implements ScanArtist/ScanAll in internal/scanner loading local/external
releases via the database layer with ctx-cancellation checks, plus a
compute-only run() hook that logs missing-release counts. Add table-driven
tests using in-memory SQLite fixtures.

cmd/naviwatcher/main_test.go

@@ -7,8 +7,54 @@ import (
 	"testing"
 
 	"naviwatcher/internal/config"
+	"naviwatcher/internal/database"
 )
 
+func TestAppRun_ScanLogsMissingReleases(t *testing.T) {
+	// Verify the compute-only run() hook scans monitored artists and returns
+	// nil without starting notifier/web. Uses an in-memory DB with one
+	// monitored artist that has one missing release.
+	db, err := database.New(":memory:")
+	if err != nil {
+		t.Fatalf("database.New() error: %v", err)
+	}
+	defer db.Close()
+
+	if err := database.SaveArtistSettings(db, &database.ArtistSettings{
+		ID:        "artist-1",
+		Name:      "Pink Floyd",
+		Monitored: true,
+	}); err != nil {
+		t.Fatalf("seed artist: %v", err)
+	}
+	if err := database.SaveLocalAlbum(db, &database.LocalAlbum{
+		ID:       "l1",
+		ArtistID: "artist-1",
+		Title:    "The Wall",
+	}); err != nil {
+		t.Fatalf("seed local album: %v", err)
+	}
+	if err := database.SaveExternalRelease(db, &database.ExternalRelease{
+		RGID:     "rg2",
+		ArtistID: "artist-1",
+		Title:    "Animals",
+	}); err != nil {
+		t.Fatalf("seed external release: %v", err)
+	}
+
+	app := &App{
+		cfg: &config.Config{Scanner: config.ScannerConfig{FuzzyThreshold: 0.85}},
+		db:  db,
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // cancel immediately so run() exits after scanning
+
+	if err := app.run(ctx); err != nil {
+		t.Fatalf("app.run() returned error: %v", err)
+	}
+}
+
 func TestConfigIntegration(t *testing.T) {
 	// Integration test: write a minimal valid config and load it via config.LoadConfig,
 	// verifying the full path that main() uses.