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.go

@@ -27,16 +27,22 @@ type Server struct {
 	// uiBaseURL is the externally reachable base URL of the dashboard (scheme +
 	// host), used to build links in notifications and elsewhere. Optional.
 	uiBaseURL string
+
+	// threshold is the fuzzy-similarity cutoff used when scanning for missing
+	// releases; 0 means use the scanner default.
+	threshold float64
 }
 
 // NewServer constructs a dashboard Server bound to the given DB and server
 // config. uiBaseURL is the externally reachable origin (e.g.
 // "http://localhost:8080") used when rendering absolute links; pass "" to omit.
-func NewServer(cfg *config.ServerConfig, db *database.DB, uiBaseURL string) *Server {
+// threshold is the fuzzy-similarity cutoff (0 → scanner default).
+func NewServer(cfg *config.ServerConfig, db *database.DB, uiBaseURL string, threshold float64) *Server {
 	s := &Server{
 		cfg:       cfg,
 		db:        db,
 		uiBaseURL: strings.TrimRight(uiBaseURL, "/"),
+		threshold: threshold,
 	}
 	mux := http.NewServeMux()
 	mux.HandleFunc("/", s.handleDashboard)