refactor(api): TOML 配置 SSOT、统一错误契约、Auth/事务加固与可观测性 (#33)

配置 SSOT（TOML + .env）
统一错误契约
Auth 与事务边界
Redis / Celery 可靠性:业务 Redis（DB/0）与 Celery broker/backend（DB/1）显式拆分；连接池、sync client
可观测性（OpenTelemetry + LGTM）

api/app/features/conversation/ws/persist.py

@@ -0,0 +1,55 @@
+"""WS pipeline 分段持久化（语音分段、按需 TTS 等独立 commit 场景）。
+
+Each helper here opens its own ``transactional()`` on the long-lived WS
+``AsyncSession`` (see ``app.core.db``): multiple commits per connection are
+intentional for incremental durability.
+
+``ConversationHistoryStore`` handles whole human/AI turn writes and refreshes
+Redis after those commits. Pipeline code may interleave ``persist.py`` segment
+commits with turn-level ``history_store`` writes; readers should treat DB as
+authoritative when cache and DB diverge briefly.
+"""
+
+from __future__ import annotations
+
+from datetime import datetime, timezone
+from typing import Optional
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.core.db import transactional
+from app.features.conversation.models import Conversation, ConversationMessage, Segment
+
+
+def mark_conversation_active(
+    conversation: Conversation, at: Optional[datetime] = None
+) -> datetime:
+    activity_time = at or datetime.now(timezone.utc)
+    conversation.last_message_at = activity_time
+    return activity_time
+
+
+async def persist_message_tts_url_segment(
+    db: AsyncSession,
+    msg: ConversationMessage,
+    segment_index: int,
+    url_stored: str,
+) -> None:
+    """按需 TTS：写入 message.tts_audio_urls[segment_index] 并提交。"""
+    urls = list(msg.tts_audio_urls or [])
+    while len(urls) <= segment_index:
+        urls.append("")
+    urls[segment_index] = url_stored
+    async with transactional(db):
+        msg.tts_audio_urls = urls
+
+
+async def persist_voice_segment_row(
+    db: AsyncSession,
+    segment: Segment,
+    conversation: Conversation,
+) -> None:
+    """语音分段入库并刷新会话活跃时间。"""
+    async with transactional(db):
+        db.add(segment)
+        mark_conversation_active(conversation)