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/artist_settings.go

@@ -4,19 +4,27 @@ import (
 	"database/sql"
 	"errors"
 	"fmt"
+	"time"
 )
 
+// DBer is the minimal query interface satisfied by both *sql.DB and *sql.Tx,
+// so callers can run statements inside or outside a transaction.
+type DBer interface {
+	Exec(query string, args ...interface{}) (sql.Result, error)
+}
+
 // GetArtistSettings retrieves an artist_settings row by ID.
 // Returns sql.ErrNoRows if the artist is not found.
 func GetArtistSettings(db *DB, id string) (*ArtistSettings, error) {
 	var (
-		s    ArtistSettings
-		mbid sql.NullString
+		s         ArtistSettings
+		mbid      sql.NullString
+		lastSynced sql.NullTime
 	)
 	err := db.Conn().QueryRow(
-		"SELECT id, name, mbid, ignore_singles, ignore_compilations, monitored FROM artist_settings WHERE id = ?",
+		"SELECT id, name, mbid, ignore_singles, ignore_compilations, monitored, last_synced FROM artist_settings WHERE id = ?",
 		id,
-	).Scan(&s.ID, &s.Name, &mbid, &s.IgnoreSingles, &s.IgnoreCompilations, &s.Monitored)
+	).Scan(&s.ID, &s.Name, &mbid, &s.IgnoreSingles, &s.IgnoreCompilations, &s.Monitored, &lastSynced)
 	if err != nil {
 		if errors.Is(err, sql.ErrNoRows) {
 			return nil, ErrArtistNotFound
@@ -24,9 +32,26 @@ func GetArtistSettings(db *DB, id string) (*ArtistSettings, error) {
 		return nil, err
 	}
 	s.MBID = mbid.String
+	if lastSynced.Valid {
+		s.LastSynced = lastSynced.Time
+	}
 	return &s, nil
 }
 
+// TouchArtistSynced records that the artist was synced at the given time. It
+// is used by the MusicBrainz pipeline to mark a successful sync (even one that
+// found zero release groups) so the cache TTL is honoured.
+func TouchArtistSynced(db DBer, artistID string, syncedAt time.Time) error {
+	_, err := db.Exec(
+		"UPDATE artist_settings SET last_synced = ? WHERE id = ?",
+		FormatCachedAt(syncedAt), artistID,
+	)
+	if err != nil {
+		return fmt.Errorf("touch artist synced: %w", err)
+	}
+	return nil
+}
+
 // SaveArtistSettings inserts or replaces an artist_settings row.
 func SaveArtistSettings(db *DB, settings *ArtistSettings) error {
 	_, err := db.Conn().Exec(