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/config/config.go

@@ -20,10 +20,11 @@ type Config struct {
 
 // ServerConfig holds HTTP server settings.
 type ServerConfig struct {
-	Host     string `yaml:"host"`
-	Port     int    `yaml:"port"`
-	Username string `yaml:"username"`
-	Password string `yaml:"password"`
+	Host      string `yaml:"host"`
+	Port      int    `yaml:"port"`
+	Username  string `yaml:"username"`
+	Password  string `yaml:"password"`
+	PublicURL string `yaml:"public_url"`
 }
 
 // NavidromeConfig holds Subsonic API connection details.