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/db/migrations/0085_library_wants.down.sql

@@ -0,0 +1,8 @@
+ALTER TABLE acquisition_settings
+    DROP COLUMN library_listing_started_at,
+    DROP COLUMN library_listed_at,
+    DROP COLUMN library_unlisted;
+
+DELETE FROM wants WHERE reason = 'library';
+ALTER TABLE wants DROP CONSTRAINT wants_reason_check;
+ALTER TABLE wants ADD CONSTRAINT wants_reason_check CHECK (reason IN ('request', 'follow', 'missing'));