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/web/server_test.go

@@ -51,7 +51,7 @@ func newServer(t *testing.T, user, pass string) (*Server, *database.DB) {
 		t.Fatalf("open db: %v", err)
 	}
 	cfg := &config.ServerConfig{Host: "0.0.0.0", Port: 8080, Username: user, Password: pass}
-	s := NewServer(cfg, db, "http://ui.example")
+	s := NewServer(cfg, db, "http://ui.example", 0)
 	return s, db
 }