fix: replace speechSynthesis announce with real audio clip via /api/tts-announce

speechSynthesis is silently muted on iOS Safari and Chrome Android after
the audio session ends (onended), so chapter announcements never played.

Fix:
- Add GET /api/tts-announce backend endpoint: streams a short TTS clip
  for arbitrary text without MinIO caching (backend/internal/backend/)
- Add GET /api/announce SvelteKit proxy route (no paywall)
- Add announceNavigatePending/announcePendingSlug/announcePendingChapter
  to AudioStore
- Rewrite onended announce branch: sets audioStore.audioUrl to the
  announcement clip URL so the persistent <audio> element plays it;
  the next onended detects announceNavigatePending and navigates
- 10s safety timeout in case the clip fails to load/end

ui/src/lib/audio.svelte.ts

@@ -160,6 +160,19 @@ class AudioStore {
 		return this.slug === slug && this.chapter === chapter;
 	}
 
+	// ── Announce-chapter navigation state ────────────────────────────────────
+	/**
+	 * When true, the <audio> element is playing a short announcement clip
+	 * (not chapter audio).  The next `onended` should navigate to
+	 * announcePendingSlug / announcePendingChapter instead of the normal
+	 * auto-next flow.
+	 */
+	announceNavigatePending = $state(false);
+	/** Target book slug for the pending announce-then-navigate transition. */
+	announcePendingSlug    = $state('');
+	/** Target chapter number for the pending announce-then-navigate transition. */
+	announcePendingChapter = $state(0);
+
 	/** Reset all next-chapter pre-fetch state. */
 	resetNextPrefetch() {
 		this.nextStatus = 'none';