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/handlers.go

@@ -32,7 +32,7 @@ func (s *Server) handleDashboard(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	threshold := s.defaultThreshold()
+	threshold := s.threshold
 	data, err := s.buildDashboardData(r.Context(), threshold)
 	if err != nil {
 		http.Error(w, fmt.Sprintf("failed to build dashboard: %v", err), http.StatusInternalServerError)
@@ -120,11 +120,14 @@ func (s *Server) buildArtistData(ctx context.Context, id string) (*ArtistData, e
 		return nil, fmt.Errorf("load local albums: %w", err)
 	}
 
-	// Compute the missing releases for this artist.
-	threshold := s.defaultThreshold()
-	missing, err := scanner.ScanAll(ctx, s.db, threshold)
+	// Compute the missing releases for this single artist (ScanArtist scopes the
+	// query to the artist instead of scanning every monitored artist). ScanAll
+	// excludes ignored releases, so every missing release surfaced here is, by
+	// definition, not ignored.
+	threshold := s.threshold
+	missing, err := scanner.ScanArtist(ctx, s.db, id, threshold)
 	if err != nil {
-		return nil, fmt.Errorf("scan: %w", err)
+		return nil, fmt.Errorf("scan artist: %w", err)
 	}
 
 	data := &ArtistData{
@@ -138,38 +141,18 @@ func (s *Server) buildArtistData(ctx context.Context, id string) (*ArtistData, e
 		data.LocalAlbums = append(data.LocalAlbums, LocalAlbumView{Title: a.Title})
 	}
 	for _, m := range missing {
-		if m.ArtistID != id {
-			continue
-		}
-		ignored, igErr := s.releaseIgnored(id, m.RGID)
-		if igErr != nil {
-			return nil, igErr
-		}
 		data.Missing = append(data.Missing, MissingReleaseView{
-			ArtistID:    id,
+			ArtistID:    m.ArtistID,
 			RGID:        m.RGID,
 			Title:       m.Title,
 			Type:        m.Type,
 			ReleaseDate: m.ReleaseDate,
-			Ignored:     ignored,
+			Ignored:     false,
 		})
 	}
 	return data, nil
 }
 
-// releaseIgnored reports whether the external release with the given RGID is
-// flagged ignored.
-func (s *Server) releaseIgnored(artistID, rgid string) (bool, error) {
-	// ScanAll already excludes ignored releases, so a missing release shown here
-	// is, by definition, not ignored. We still surface the persisted flag so the
-	// UI can reflect a release that was ignored and later re-evaluated.
-	rel, err := database.GetExternalRelease(s.db, rgid)
-	if err != nil {
-		return false, fmt.Errorf("get external release %s: %w", rgid, err)
-	}
-	return rel.IsIgnored, nil
-}
-
 // ArchiveData is the view model for the ignored-releases archive page.
 type ArchiveData struct {
 	Releases []MissingReleaseView
@@ -208,20 +191,20 @@ func (s *Server) handleArchive(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
-// defaultThreshold returns the fuzzy threshold to use when scanning for the
-// dashboard. It is currently fixed at the engine default; later wiring can
-// thread the configured threshold through the Server if desired.
-func (s *Server) defaultThreshold() float64 {
-	return 0 // 0 → scanner.DefaultThreshold
-}
-
 // NewServerWithConfig is a convenience constructor that accepts the full
 // *config.Config (mirroring how the app constructs other components). It
-// forwards the server sub-config and derives uiBaseURL from host/port.
+// forwards the server sub-config and derives uiBaseURL from the configured
+// public_url, falling back to a best-effort host:port.
 func NewServerWithConfig(cfg *config.Config, db *database.DB) *Server {
-	// Build a best-effort external base URL from the server config.
-	base := fmt.Sprintf("http://%s:%d", cfg.Server.Host, cfg.Server.Port)
-	return NewServer(&cfg.Server, db, base)
+	// Prefer an explicit, externally-reachable public_url (e.g. behind a
+	// reverse proxy). Fall back to host:port — but if the bind host is the
+	// unspecified "0.0.0.0", it is not reachable from outside the host, so
+	// omit the link rather than advertise an unusable address.
+	base := cfg.Server.PublicURL
+	if base == "" && cfg.Server.Host != "0.0.0.0" && cfg.Server.Host != "" {
+		base = fmt.Sprintf("http://%s:%d", cfg.Server.Host, cfg.Server.Port)
+	}
+	return NewServer(&cfg.Server, db, base, cfg.Scanner.FuzzyThreshold)
 }
 
 // ignoreOrRestore handles the POST /artist/{id}/ignore and .../restore routes.
@@ -290,8 +273,3 @@ func (s *Server) toggleIgnoreSingles(w http.ResponseWriter, r *http.Request) {
 	http.Redirect(w, r, "/artist/"+id, http.StatusSeeOther)
 }
 
-// endsWith reports whether s ends with suffix.
-func endsWith(s, suffix string) bool {
-	return len(s) >= len(suffix) && s[len(s)-len(suffix):] == suffix
-}
-