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/querier.go

@@ -202,6 +202,8 @@ type Querier interface {
 	GetAllSystemSettings(ctx context.Context) ([]GetAllSystemSettingsRow, error)
 	GetAllSystemSettingsFull(ctx context.Context) ([]SystemSettings, error)
 	GetAnnotationsForBook(ctx context.Context, arg GetAnnotationsForBookParams) ([]GetAnnotationsForBookRow, error)
+	// Per-book playback speed preference (player plan §8).
+	GetAudioPlaybackSpeed(ctx context.Context, arg GetAudioPlaybackSpeedParams) (float32, error)
 	// Lifetime session rows for ONE book (the book-detail stats card).
 	GetBookReadingSessions(ctx context.Context, arg GetBookReadingSessionsParams) ([]GetBookReadingSessionsRow, error)
 	GetBooksByTag(ctx context.Context, arg GetBooksByTagParams) ([]MediaItems, error)
@@ -696,12 +698,14 @@ type Querier interface {
 	UpdateSystemSetting(ctx context.Context, arg UpdateSystemSettingParams) error
 	// Update universal progress
 	UpdateUniversalProgress(ctx context.Context, arg UpdateUniversalProgressParams) (ReadingProgress, error)
+	UpdateUserDefaultAudioSpeed(ctx context.Context, arg UpdateUserDefaultAudioSpeedParams) error
 	UpdateUserMaxDevices(ctx context.Context, arg UpdateUserMaxDevicesParams) (Users, error)
 	UpdateUserProfile(ctx context.Context, arg UpdateUserProfileParams) error
 	UpdateUserRole(ctx context.Context, arg UpdateUserRoleParams) (UpdateUserRoleRow, error)
 	UpdateUserTheme(ctx context.Context, arg UpdateUserThemeParams) error
 	UpdateUserTimezone(ctx context.Context, arg UpdateUserTimezoneParams) error
 	UpdateUsername(ctx context.Context, arg UpdateUsernameParams) error
+	UpsertAudioPlaybackSpeed(ctx context.Context, arg UpsertAudioPlaybackSpeedParams) (float32, error)
 	UpsertDashboardPreferences(ctx context.Context, arg UpsertDashboardPreferencesParams) (UserDashboardPreferences, error)
 	UpsertGlobalDashboardPreferences(ctx context.Context, arg UpsertGlobalDashboardPreferencesParams) (UserDashboardPreferencesGlobal, error)
 	UpsertMediaReview(ctx context.Context, arg UpsertMediaReviewParams) (MediaReviews, error)