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.go

@@ -12,6 +12,7 @@ import (
 	"naviwatcher/internal/config"
 	"naviwatcher/internal/database"
 	"naviwatcher/internal/musicbrainz"
+	"naviwatcher/internal/scanner"
 )
 
 // App holds all application dependencies for clean shutdown and testability.
@@ -91,9 +92,27 @@ func (a *App) Close() {
 }
 
 func (a *App) run(ctx context.Context) error {
+	// Compute-only scanner hook: scan all monitored artists for missing
+	// releases and log the count. Notifier/Web UI are out of scope for this
+	// plan, so results are only logged. This call is non-blocking and
+	// goroutine-safe; it observes ctx cancellation and returns early.
+	missing, err := scanner.ScanAll(ctx, a.db, a.cfg.Scanner.FuzzyThreshold)
+	if err != nil {
+		if ctx.Err() != nil {
+			// Context cancelled (e.g. shutdown) — exit cleanly.
+			return nil
+		}
+		return fmt.Errorf("scan all: %w", err)
+	}
+
+	log.Printf("Scan complete: %d missing release(s) across monitored artists", len(missing))
+	for _, m := range missing {
+		log.Printf("  missing: artist=%s rgid=%s title=%q", m.ArtistID, m.RGID, m.Title)
+	}
+
 	// Main application loop — blocks until context is cancelled.
-	// Business logic (scanner, notifier, web server) will be wired into
-	// separate goroutines here in future tasks.
+	// Business logic (notifier, web server) will be wired into separate
+	// goroutines here in future tasks.
 	<-ctx.Done()
 	return nil
 }