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/audio_reader.templ

@@ -403,9 +403,19 @@ templ AudioReader(user User, metadata ReaderMetadata) {
 								class="w-full text-left px-3 py-1.5 rounded-md text-sm"
 								:class="Number(speed) === rate ? 'font-semibold' : ''"
 								:style="Number(speed) === rate ? 'background-color: var(--accent-muted); color: var(--text-primary);' : 'color: var(--text-secondary);'"
-								x-text="rate + '×'"
+								x-text="rate + '×' + (isDefaultSpeed(rate) ? ' · default' : '')"
 							></button>
 						</template>
+						<!-- §8: pin the current rate as the user default —
+						     books without their own speed pick it up -->
+						<button
+							type="button"
+							x-show="!isDefaultSpeed(speed)"
+							@click="setDefaultSpeed(speed); speedOpen = false"
+							class="w-full text-left px-3 py-1.5 mt-1 rounded-md text-xs border-t"
+							style="border-color: var(--border); color: var(--text-secondary);"
+							x-text="'Make ' + speed + '× my default'"
+						></button>
 					</div>
 				</div>
 				<button