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/alembic/versions/0018_users_language_preference.py

@@ -0,0 +1,42 @@
+"""users：language_preference 字段
+
+Revision ID: 0018_users_language_preference
+Revises: 0017_segment_narrative_defer
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+
+from alembic import op
+
+revision: str = "0018_users_language_preference"
+down_revision: Union[str, None] = "0017_segment_narrative_defer"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def _column_names(table_name: str) -> set[str]:
+    bind = op.get_bind()
+    inspector = sa.inspect(bind)
+    return {column["name"] for column in inspector.get_columns(table_name)}
+
+
+def upgrade() -> None:
+    columns = _column_names("users")
+    if "language_preference" not in columns:
+        op.add_column(
+            "users",
+            sa.Column(
+                "language_preference",
+                sa.String(length=8),
+                nullable=False,
+                server_default=sa.text("'zh'"),
+            ),
+        )
+
+
+def downgrade() -> None:
+    columns = _column_names("users")
+    if "language_preference" in columns:
+        op.drop_column("users", "language_preference")