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/tests/test_memoir_skip_story.py

@@ -134,6 +134,7 @@ def test_prepare_batches_batch_llm_path_matches_per_segment_skip_logic(
         *,
         chunk_size: int = 24,
         on_chunk=None,
+        language: str = "zh",
     ) -> dict:
         return {
             "mix-1": BatchPhase1SegmentRow(