feat(audio-player): per-book playback speed + user default (plan §8)

Speed becomes a per-USER-per-BOOK preference that syncs across
devices, with a user-level default for books not yet played:
- new audio_playback_speed table (mirrors reading_speed's shape; a
  preference, NOT a measurement and NOT a reading_progress column) +
  users.default_audio_speed, schema in place
- chapters endpoint resolves per-book row → user default → 1.0 and
  ships playback_speed + default_playback_speed (zero extra round
  trips; default powers the dropdown marker)
- PUT /api/media-items/:id/audio-speed upserts (range-checked; the
  client SPEEDS list stays the real gate; library visibility gates
  writes)
- the user default rides the profile path like theme/timezone:
  profile form gains a 'Default Playback Speed' select (server-side
  SpeedOptions mirrors the web SPEEDS list), GET /profile carries it
- web: restorePosition applies the resolved server speed (localStorage
  audio_speed demotes to a first-paint cache); setSpeed PUTs
  fire-and-forget with a failure toast; the Speed dropdown gains a
  '(default)' row marker and a 'Make N× my default' action with a
  success toast

ALSO fixes a pre-existing profile-page bug surfaced by the §8 form
verification: the password section's current_password input was
required INSIDE the shared form, so HTML5 validation blocked every
Save Changes submission (dead since the shell migration). The
password flow keeps its own gates (disabled-button checkers + server
400).

Integration test TestAudioPlaybackSpeed pins the full contract (unset
→ 1x, upsert resolution, range rejection, default fallback for
row-less books, default updates re-satisfying unset books). Verified
live: 9 Playwright checks.

internal/database/queries/queries.sql

@@ -28,6 +28,7 @@ SELECT
     u.created_at,
     u.updated_at,
     u.timezone,
+    u.default_audio_speed,
     (SELECT COUNT(*) FROM devices WHERE user_id = u.id) as device_count
 FROM users u
 WHERE u.id = $1;
@@ -511,6 +512,22 @@ RETURNING id, email, username, role;
 -- name: UpdateUserTimezone :exec
 UPDATE users SET timezone = $2, updated_at = NOW() WHERE id = $1;
 
+-- name: UpdateUserDefaultAudioSpeed :exec
+UPDATE users SET default_audio_speed = $2, updated_at = NOW() WHERE id = $1;
+
+-- Per-book playback speed preference (player plan §8).
+
+-- name: GetAudioPlaybackSpeed :one
+SELECT speed FROM audio_playback_speed
+WHERE user_id = $1 AND media_item_id = $2;
+
+-- name: UpsertAudioPlaybackSpeed :one
+INSERT INTO audio_playback_speed (user_id, media_item_id, speed)
+VALUES ($1, $2, $3)
+ON CONFLICT (user_id, media_item_id)
+DO UPDATE SET speed = EXCLUDED.speed, updated_at = NOW()
+RETURNING speed;
+
 -- name: GetSystemTimezone :one
 SELECT setting_value FROM system_settings WHERE setting_key = 'default_timezone';