feat(library): fold a missing track into its on-disk replacement (M485 #5286 #5287)

A track marked missing whose replacement is already on disk under another
row — on the operator's library 355 of 451 missing tracks, nearly all Lidarr
mp3 -> flac upgrades — is folded into the replacement: likes, plays, playlist
entries and tags move across and the missing row goes. Move adoption could
not catch these: the replacements were re-encodes (no shared audio hash) with
no recording MBID at import.

Pairs (ListMissingTrackPairs): the same recording MBID within the album
group, or the same album row and title ignoring case. Each side must have
exactly one candidate; conflicting MBIDs refuse a pair. No duration or track
position gate: on the 142 pairs known to be one recording, 18% differed by
over 2s and the poorly tagged set is where numbering is broken (spike #5274).

Each pair folds in its own transaction after locking both rows and checking
the pair still holds. Runs automatically (operator, 2026-10-07) after a full
scan, after a watcher batch that added or updated tracks, and after an
AcoustID pass that matched any track. The first scan after deploy repairs the
existing rows.

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

internal/db/queries/missing_pairs.sql

@@ -0,0 +1,75 @@
+-- Missing-pair pass (M485). A track marked missing whose replacement is already
+-- on disk under another row — most often a Lidarr quality upgrade, mp3 → flac at
+-- a new path — is folded into that replacement, so its history moves across and
+-- re-acquisition stops chasing music the library already has.
+--
+-- Move adoption (#2528) cannot catch these: it links a new file to a missing row
+-- at the moment the file is first inserted, and the replacement arrives as a
+-- re-encode with no recording MBID, so it has nothing to match on. AcoustID may
+-- give it an MBID later; by then both rows exist and only a fold can join them.
+
+-- name: ListMissingTrackPairs :many
+-- Every candidate (missing, present) pair, with how many candidates each side
+-- has. Two rules, measured on the operator's library on 2026-10-07 (#5274):
+--
+--   mbid   the same recording MBID, within the album group (the album row, or
+--          albums sharing a release group). 142 of 359 pairs.
+--   title  the same album row and the same title, ignoring case. 213 more. The
+--          replacements are often not the original file — another edition,
+--          re-encoded, untagged — so the metadata is what is left to go on.
+--
+-- Deliberately NOT gated on duration or track position. On the 142 pairs known
+-- to be one recording, 18% differed by over 2s (old mp3 durations are poor
+-- estimates), and the poorly tagged set is exactly where numbering is broken.
+--
+-- A pair whose MBIDs are both set and differ is not a title match: the tags say
+-- they are different recordings. The caller folds a pair only when each side has
+-- exactly one candidate; anything else is ambiguous and left alone.
+WITH grp AS (
+  SELECT id AS album_id, COALESCE(release_group_mbid, id::text) AS g
+    FROM albums
+),
+cand AS (
+  SELECT m.id AS missing_id, p.id AS present_id, 'mbid'::text AS rule
+    FROM tracks m
+    JOIN grp gm ON gm.album_id = m.album_id
+    JOIN grp gp ON gp.g = gm.g
+    JOIN tracks p ON p.album_id = gp.album_id
+                 AND p.missing_since IS NULL
+                 AND p.mbid = m.mbid
+   WHERE m.missing_since IS NOT NULL
+     AND m.mbid IS NOT NULL
+  UNION ALL
+  SELECT m.id, p.id, 'title'::text
+    FROM tracks m
+    JOIN tracks p ON p.album_id = m.album_id
+                 AND p.missing_since IS NULL
+                 AND lower(p.title) = lower(m.title)
+   WHERE m.missing_since IS NOT NULL
+     AND (m.mbid IS NULL OR p.mbid IS NULL OR m.mbid = p.mbid)
+),
+pairs AS (
+  -- A pair both rules found is one candidate, credited to the stronger rule.
+  SELECT missing_id, present_id, min(rule)::text AS rule
+    FROM cand
+   GROUP BY missing_id, present_id
+)
+SELECT pr.missing_id, pr.present_id, pr.rule,
+       m.file_path AS missing_path, p.file_path AS present_path,
+       (count(*) OVER (PARTITION BY pr.missing_id))::int AS missing_candidates,
+       (count(*) OVER (PARTITION BY pr.present_id))::int AS present_candidates
+  FROM pairs pr
+  JOIN tracks m ON m.id = pr.missing_id
+  JOIN tracks p ON p.id = pr.present_id
+ ORDER BY m.file_path, p.file_path;
+
+-- name: LockTracksForPairing :many
+-- Locks both rows of a pair for the fold's transaction, in id order so two
+-- passes cannot deadlock, and returns what the fold re-checks: the missing row
+-- must still be missing and the present one still present. A scan may have
+-- restored or adopted the first, or marked the second, since the pairs were listed.
+SELECT id, missing_since
+  FROM tracks
+ WHERE id = ANY(sqlc.arg(ids)::uuid[])
+ ORDER BY id
+   FOR UPDATE;