feat(library): album loudness from the tracks' summed block histograms (M464 #4996)

Album-mode normalization plays a whole album at one gain. That gain comes
from album_loudness (migration 0066): BS.1770's gated loudness over every
block on the album, computed by summing the tracks' stored histograms and
gating the sum. No audio is decoded again. Album true peak is the loudest
track's.

- Recomputed by the loudness worker each tick, after the track pass and
  whether or not analysis is switched on. ListAlbumsNeedingLoudness lists
  albums whose md5 over (present track id, measurement version and time) no
  longer matches the stored digest. One comparison covers every way
  membership changes (scan retag, duplicate merge, delete, missing and
  restored) without hooking each.
- No album value until every present track has a settled measurement, so
  an album's gain doesn't shift mid-listen as the rest is measured. Silent
  and unreadable tracks count as settled.
- Rows for albums with no present track left are dropped.
- The parser and the merge share trimBins.

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

internal/db/queries/loudness.sql

@@ -75,3 +75,69 @@ UPDATE loudness_settings
        updated_at           = now()
  WHERE id = true
 RETURNING *;
+
+-- name: ListAlbumsNeedingLoudness :many
+-- The album pass's work queue (#4996): albums whose present tracks or their
+-- measurements have changed since album loudness was last computed, or that
+-- never had it. The digest is over every present track's id and the
+-- measurement it holds at the current version ('-' for none), so a track
+-- joining, leaving or being re-measured changes it. Keyset-paged on album id
+-- so a pass ends even if storing one album keeps failing.
+WITH present AS (
+    SELECT t.album_id,
+           md5(string_agg(
+               t.id::text || ':' || coalesce(
+                   l.analysis_version::text || '@' || l.analyzed_at::text, '-'),
+               ',' ORDER BY t.id)) AS digest
+      FROM tracks t
+      LEFT JOIN track_loudness l
+             ON l.track_id = t.id AND l.analysis_version >= sqlc.arg(current_version)::smallint
+     WHERE t.missing_since IS NULL
+     GROUP BY t.album_id
+)
+SELECT c.album_id, c.digest::text AS digest
+  FROM present c
+  LEFT JOIN album_loudness a ON a.album_id = c.album_id
+ WHERE (a.album_id IS NULL OR a.inputs_digest <> c.digest)
+   -- Casts: sqlc cannot infer a parameter's type through a CTE alias.
+   AND c.album_id > sqlc.arg(after_id)::uuid
+ ORDER BY c.album_id
+ LIMIT sqlc.arg(batch_limit)::integer;
+
+-- name: ListAlbumLoudnessInputs :many
+-- Every present track on one album with its current measurement, if any.
+-- settled is false for a track not yet measured at the current version.
+SELECT t.id,
+       (l.track_id IS NOT NULL)::boolean AS settled,
+       l.true_peak_dbtp,
+       l.block_hist_start,
+       l.block_hist
+  FROM tracks t
+  LEFT JOIN track_loudness l
+         ON l.track_id = t.id AND l.analysis_version >= sqlc.arg(current_version)
+ WHERE t.album_id = sqlc.arg(album_id)
+   AND t.missing_since IS NULL;
+
+-- name: UpsertAlbumLoudness :exec
+INSERT INTO album_loudness (
+    album_id, integrated_lufs, true_peak_dbtp, tracks_total, tracks_settled, inputs_digest
+) VALUES (
+    sqlc.arg(album_id), sqlc.narg(integrated_lufs), sqlc.narg(true_peak_dbtp),
+    sqlc.arg(tracks_total), sqlc.arg(tracks_settled), sqlc.arg(inputs_digest)
+)
+ON CONFLICT (album_id) DO UPDATE SET
+    integrated_lufs = EXCLUDED.integrated_lufs,
+    true_peak_dbtp  = EXCLUDED.true_peak_dbtp,
+    tracks_total    = EXCLUDED.tracks_total,
+    tracks_settled  = EXCLUDED.tracks_settled,
+    inputs_digest   = EXCLUDED.inputs_digest,
+    computed_at     = now();
+
+-- name: DeleteOrphanAlbumLoudness :execrows
+-- Album rows whose album has no present track left (every file missing). The
+-- album row itself survives a missing file, so its loudness would otherwise
+-- stay behind describing tracks that are gone; it is recomputed if they return.
+DELETE FROM album_loudness a
+ WHERE NOT EXISTS (
+     SELECT 1 FROM tracks t WHERE t.album_id = a.album_id AND t.missing_since IS NULL
+ );