Add refresh from source for entries and whole lists

- Every provider can look an entry up again by its ID (TMDB, TheTVDB,
  IGDB, Deezer, MusicBrainz, Wikipedia); search and lookup share parsing
- POST /api/items/{id}/refresh re-applies title, subtitle, year,
  description, link, details and artwork. Kept: the "why" note, rank,
  theater production/venue/cast and a theater entry's year, the Deezer
  preview match. Empty source fields don't erase existing values
- New items.image_custom column (migration 4; existing uploads marked)
  records artwork the owner chose; refresh keeps it unless asked
- Refused for custom lists and hand-added entries; entries the source
  dropped are reported, and a refresh whose title shares no words with
  the current one is rejected as a likely bad ID
- Editor: refresh button per row (asks about the owner's own artwork),
  and "Refresh all" with a confirm step, replace-my-art checkbox,
  progress with Stop, and a summary of problems

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/providers/tmdb.go

@@ -144,3 +144,37 @@ func (t *tmdb) Details(ctx context.Context, res Result) (Result, error) {
 	}
 	return res, nil
 }
+
+// Lookup refreshes one movie or show ("movie:123" / "tv:456") by its TMDB ID.
+func (t *tmdb) Lookup(ctx context.Context, providerID string) (Result, error) {
+	kind, id, _ := strings.Cut(providerID, ":")
+	if kind != "movie" && kind != "tv" {
+		return Result{}, ErrGone
+	}
+	if _, err := strconv.ParseInt(id, 10, 64); err != nil {
+		return Result{}, ErrGone
+	}
+	var h tmdbHit
+	err := t.get(ctx, "/"+kind+"/"+id, nil, &h)
+	if isStatus(err, 404) {
+		return Result{}, ErrGone
+	}
+	if err != nil {
+		return Result{}, err
+	}
+	title, date := h.Title, h.ReleaseDate
+	if kind == "tv" {
+		title, date = h.Name, h.FirstAirDate
+	}
+	res := Result{
+		Provider:    "tmdb",
+		ProviderID:  providerID,
+		Title:       title,
+		Year:        yearOf(date),
+		ImageURL:    tmdbImage(h.PosterPath),
+		ExternalURL: "https://www.themoviedb.org/" + kind + "/" + id,
+		Overview:    truncate(h.Overview, maxDescription),
+	}
+	d := &tmdb{r: t.r, key: t.key, kind: kind}
+	return d.Details(ctx, res)
+}