feat: implement rate limiting and caching layer for MusicBrainz provider

- Add golang.org/x/time/rate dependency for token-bucket rate limiting
- Replace custom channel-based rate limiter with rate.NewLimiter(1, 1)
- Add context.Context support to doGet for cancellation
- Add cached_at column to external_releases via migration 005
- Implement cache hit/miss queries with TTL-based filtering
- Add CacheStats type for tracking cached RGIDs
- Update ExternalRelease struct with CachedAt field
- Add rate limiting tests (1 req/sec enforcement, burst behavior)
- Add cache tests (hit, miss, expired, mixed, empty artist)
- Update migration count test for new migration

internal/database/database_test.go

@@ -205,8 +205,8 @@ func TestMigrationTracking(t *testing.T) {
 		t.Fatalf("query migrations count: %v", err)
 	}
 
-	// We have 4 recorded migrations: artist_settings, external_releases, local_albums, notifications_sent.
-	if count != 4 {
-		t.Errorf("expected 4 applied migrations, got %d", count)
+	// We have 5 recorded migrations: artist_settings, external_releases, local_albums, notifications_sent, cached_at column.
+	if count != 5 {
+		t.Errorf("expected 5 applied migrations, got %d", count)
 	}
 }