fix: address code review findings

- Fix duplicate Telegram notifications: SyncArtistDiscography no longer wipes
  notifications_sent for the whole artist on every cache-miss re-sync; only
  markers for releases that disappear are pruned (FK-safe via INSERT OR REPLACE
  + rgid NOT IN (...)).
- Cache empty MusicBrainz discographies via a new artist_settings.last_synced
  column (migration 009) so zero-release artists honor the TTL instead of being
  re-fetched every cycle.
- Wire the Web UI server and Telegram notifier scheduler into main.run/NewApp.
- Guard startPeriodicSync against overlapping syncs with a done-channel slot.
- Add server.public_url config; NewServerWithConfig derives reachable links
  and no longer advertises the 0.0.0.0 bind address.
- Web handlers: use scanner.ScanArtist per artist, drop always-false
  releaseIgnored lookup and dead endsWith, thread configured threshold.
- Limit :memory: DB pool to one connection so migrations and queries share the
  same in-memory store.

internal/database/database_test.go

@@ -205,11 +205,11 @@ func TestMigrationTracking(t *testing.T) {
 		t.Fatalf("query migrations count: %v", err)
 	}
 
-	// We have 8 recorded migrations: artist_settings, external_releases,
+	// We have 9 recorded migrations: artist_settings, external_releases,
 	// local_albums, notifications_sent, cached_at column, secondary_types
-	// column, the external_releases.artist_id index, and the artist_settings
-	// mbid column.
-	if count != 8 {
-		t.Errorf("expected 8 applied migrations, got %d", count)
+	// column, the external_releases.artist_id index, the artist_settings mbid
+	// column, and the artist_settings last_synced column.
+	if count != 9 {
+		t.Errorf("expected 9 applied migrations, got %d", count)
 	}
 }