feat: per-user volume leveling preference, synced across devices (M464 #4998)

Mode (off, auto, track, album), target (-18, -16, -14 LUFS) and boost
(within headroom, or fully with a limiter), stored per user on the server
so the web player, the Android app and casts apply the same one.

- Server: user_normalization_prefs (migration 0067), GET/PUT
  /api/me/normalization; a whole-body PUT, validated, last write wins.
- Web: Settings > Playback > Volume leveling. Saves at once, restores the
  old choice if the save fails, and caches the value for the player.
- Android: Settings card. The device keeps a copy for offline playback
  (Room v9 with an explicit migration, so the upgrade wipes nothing).
  Writes are offline-first: shown at once, PUT best effort, queued on
  failure (NORMALIZATION_SET, collapsed to the newest). A refresh never
  overwrites a change still queued.

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

internal/db/migrations/0067_user_normalization_prefs.up.sql

@@ -0,0 +1,22 @@
+-- Each user's loudness-normalization preference (M464 #4998). It is kept on
+-- the server, not per device, so the web player, the Android app and the
+-- leveled stream for UPnP/Sonos all apply the same one.
+--
+-- mode:   off, auto (album gain while an album plays in order, otherwise
+--         track gain), track or album.
+-- target: the loudness tracks are leveled to, in LUFS.
+-- boost:  headroom raises quiet tracks only as far as their true peak
+--         allows; limiter raises them all the way and limits the peaks.
+--
+-- No row means the defaults, which are the column defaults here and
+-- library.DefaultNormalizationPrefs in Go (a test holds them equal).
+CREATE TABLE user_normalization_prefs (
+    user_id     uuid PRIMARY KEY REFERENCES users(id) ON DELETE CASCADE,
+    mode        text NOT NULL DEFAULT 'auto'
+                CHECK (mode IN ('off', 'auto', 'track', 'album')),
+    target_lufs smallint NOT NULL DEFAULT -18
+                CHECK (target_lufs IN (-18, -16, -14)),
+    boost       text NOT NULL DEFAULT 'headroom'
+                CHECK (boost IN ('headroom', 'limiter')),
+    updated_at  timestamptz NOT NULL DEFAULT now()
+);