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.

config.yaml.example

@@ -4,6 +4,11 @@ server:
   # Basic Auth for Web UI access
   username: "admin"
   password: "CHANGE_ME"
+  # Externally-reachable base URL for links in Telegram digests (e.g. behind a
+  # reverse proxy). If omitted, links are derived from host:port — but when host
+  # is 0.0.0.0 (the unspecified bind address) no link is emitted, since it is
+  # not reachable from outside the host.
+  public_url: "https://naviwatcher.example.com"
 
 navidrome:
   url: "http://localhost:4533"