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/migrations/0066_album_loudness.up.sql

@@ -0,0 +1,31 @@
+-- 0066_album_loudness.up.sql — album loudness for album-mode normalization
+-- (Scribe milestone #464, #4996).
+--
+-- An album's loudness is the gated loudness of every 400 ms block on the album,
+-- not an average of its tracks' values: a quiet interlude and a loud single
+-- should keep their difference when the album plays in order. It is computed
+-- from the per-track block histograms in track_loudness (0065), summed, so no
+-- audio is decoded again.
+--
+-- A derived value, recomputed by the loudness worker whenever its inputs
+-- change. inputs_digest is an md5 over the album's present tracks and the
+-- measurement each holds; the worker recomputes every album whose stored digest
+-- no longer matches. That one comparison covers every way membership changes
+-- (a scan moving a track between albums, a duplicate merge, a delete, a file
+-- going missing or coming back) without hooking each of them.
+CREATE TABLE album_loudness (
+    album_id        uuid        PRIMARY KEY REFERENCES albums (id) ON DELETE CASCADE,
+    -- NULL until every present track has a settled measurement: an album
+    -- leveled from half its tracks would jump when the rest arrived. Also NULL
+    -- for an album with no block above the gate. Clients fall back to track
+    -- gain while it is NULL.
+    integrated_lufs real,
+    -- The loudest true peak of any track on the album, so album gain is held
+    -- to the headroom of its loudest track.
+    true_peak_dbtp  real,
+    tracks_total    integer     NOT NULL,
+    -- Tracks with a settled measurement (measured, silent or unreadable).
+    tracks_settled  integer     NOT NULL,
+    inputs_digest   text        NOT NULL,
+    computed_at     timestamptz NOT NULL DEFAULT now()
+);