feat(i18n): persist language preference and thread through chat, memoir, TTS

- Add users.language_preference (Alembic 0018, default zh); capture at signup/SMS
  only; expose on auth and profile APIs
- Lite English prompts for chat and memoir; localized stage labels and agent
  names (Life Echo / 岁月知己)
- Tencent TTS: language-aware synthesis, ModelType=1 for 501004, English chunking
- WebSocket pipeline: emit all AGENT_RESPONSE segments when TTS cancels; INFO logs
  for tts_this_turn and TTS decisions; on-demand TTS logging
- Expo: device language on auth, i18n tiers/agent name, [SPLIT] streaming UX fixes
- Tests for migration, prompts, pipeline, router tts_this_turn, reply segments

Co-authored-by: Cursor <cursoragent@cursor.com>

app-expo/src/i18n/locales/en/profile.json

@@ -64,6 +64,12 @@
   },
   "signingOut": "Signing out...",
   "signOut": "Sign Out",
+  "tier": {
+    "free": "Free",
+    "pro": "Pro",
+    "pro_plus": "Pro+",
+    "test": "Test"
+  },
   "userNamePlaceholder": "User",
   "userTier": "{{tier}}"
 }