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.

templates/types.go

@@ -17,8 +17,15 @@ type User struct {
 	CreatedAt time.Time
 	Token     string
 	Timezone  string
+	// User-level default audiobook playback speed (plan §8); 0 means
+	// "unset" and renders as the 1× option.
+	DefaultAudioSpeed float64
 }
 
+// SpeedOption is one entry of the default-playback-speed select — the
+// server-side mirror of the web player's SPEEDS list (plan §5/§8).
+var SpeedOptions = []float64{0.5, 0.75, 1, 1.25, 1.5, 1.75, 2, 2.5, 3}
+
 type PageData struct {
 	User        User
 	CurrentPage string