feat: the library's albums go on the wanted list as held, once; after that the list is the truth (M502 #5567)

A one-time job in the wanted worker lists every album in the library as a
held want, one per release group, with a record that it happened
(acquisition_settings.library_listed_at). Albums whose tags name only a
release are named on MusicBrainz first, 120 a pass; what nothing names is
counted and shown on the Held tab.

Re-acquisition now wants back only albums on the list. Taking a held album
off the list leaves its files, and it is never fetched back; albums put in
the library folder by hand afterwards are not added.

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

internal/api/admin_wanted.go

@@ -86,6 +86,16 @@ type wantsPage struct {
 	Limit  int        `json:"limit"`
 	Offset int        `json:"offset"`
 	Counts wantCounts `json:"counts"`
+	// Library is the one-time listing of the library's albums (#5567).
+	Library libraryListingView `json:"library"`
+}
+
+// libraryListingView: whether the library's albums are all on the list
+// yet, and how many could not be listed (nothing names them to
+// MusicBrainz).
+type libraryListingView struct {
+	ListedAt *string `json:"listed_at"`
+	Unlisted int32   `json:"unlisted"`
 }
 
 const wantsPageSize = 50
@@ -116,11 +126,21 @@ func (h *handlers) handleListWants(w http.ResponseWriter, r *http.Request) {
 		writeErrWithLog(w, h.logger, "admin wanted: count", apierror.Internal(err))
 		return
 	}
+	listing, err := q.GetLibraryListing(r.Context())
+	if err != nil {
+		writeErrWithLog(w, h.logger, "admin wanted: library listing", apierror.Internal(err))
+		return
+	}
 	out := wantsPage{
-		Items: make([]wantView, 0, len(rows)), Limit: wantsPageSize, Offset: offset,
+		Library: libraryListingView{Unlisted: listing.LibraryUnlisted},
+		Items:   make([]wantView, 0, len(rows)), Limit: wantsPageSize, Offset: offset,
 		Counts: wantCounts{Wanted: counts.Wanted, Missing: counts.Missing, Held: counts.Held, Following: counts.Following,
 			Activity: counts.Activity, Attention: counts.Attention, Blocked: counts.Blocked},
 	}
+	if listing.LibraryListedAt.Valid {
+		at := formatTimestamp(listing.LibraryListedAt)
+		out.Library.ListedAt = &at
+	}
 	switch state {
 	case "missing":
 		out.Total = counts.Missing