fix: same-titled albums of two release groups are two albums, and a missing track with no disc is on disc 1

The scanner found an album by artist and title alone, so Weezer's six
self-titled albums (1994–2026) were one 118-track row and their merges,
wants and upgrades counted them as one. A same-named row whose release group
is known and differs from the file's is now another album; tagReadVersion 7
re-reads the library so existing tracks move to their own rows.

WantHoldings compared discs with IS NOT DISTINCT FROM, so a missing track 6
with no disc was never covered by a present disc 1 track 6. Air's Love 2
stayed wanted and was downloaded again for nothing.

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

internal/db/queries/wanted.sql

@@ -146,11 +146,13 @@ SELECT a.id AS album_id,
        COALESCE(bool_or(t.missing_since IS NOT NULL AND t.missing_since < sqlc.arg(since)::timestamptz), false)::bool AS missing_before,
        -- Missing tracks no track on disk stands in for, by position or
        -- title. A missing row whose song is back at a new path (#5245) is
-       -- covered; a lost bonus track is not.
+       -- covered; a lost bonus track is not. A track with no disc number is
+       -- on disc 1: Air's Love 2 stayed wanted, and was downloaded again for
+       -- nothing, over a missing track 6 with no disc beside a present 1/6.
        COUNT(*) FILTER (WHERE t.missing_since IS NOT NULL AND NOT EXISTS (
            SELECT 1 FROM tracks p
             WHERE p.album_id = a.id AND p.missing_since IS NULL
-              AND ((p.track_number = t.track_number AND p.disc_number IS NOT DISTINCT FROM t.disc_number)
+              AND ((p.track_number = t.track_number AND COALESCE(p.disc_number, 1) = COALESCE(t.disc_number, 1))
                    OR lower(p.title) = lower(t.title))))::int AS uncovered_missing
   FROM albums a
   JOIN tracks t ON t.album_id = a.id