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>

api/app/ports/tts.py

@@ -5,6 +5,11 @@ from typing import Protocol, runtime_checkable
 
 @runtime_checkable
 class TTSProvider(Protocol):
-    async def synthesize(self, text: str, voice: str = "alloy") -> bytes:
-        """Convert text to speech audio bytes."""
+    async def synthesize(
+        self, text: str, voice: str = "alloy", *, language: str = "zh"
+    ) -> bytes:
+        """Convert text to speech audio bytes.
+
+        language: 'zh' or 'en'. Adapters that natively detect language may ignore it.
+        """
         ...