修复语音/通话只保存最后一轮对话：改为逐轮创建 ChatLog 记录

原来 _append_voice_log_chunk 对同一个 conversation_id 只维护一条 ChatLog，
多轮对话的 user_msg 和 ai_msg 被反复覆盖，最终只剩最后一轮。

修改为：
- 每次新用户消息（asr_result）都创建新的 ChatLog 记录，ID 存入 session_state
- AI 回复（chat_response）通过 current_log_id 找到当前轮次的记录并更新
- 会话结束时 _persist_voice_session_history 也通过 current_log_id 更新最后一轮

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/api/voice_chat_router.py

@@ -797,6 +797,16 @@ async def _persist_voice_session_history(
             )
             main_log = existing.scalar_one_or_none()
 
+            # 如果有当前轮次的日志，更新它；否则更新最后一轮
+            current_log_id = session_state.get("current_log_id")
+            if current_log_id:
+                current_result = await db.execute(
+                    select(ChatLog).where(ChatLog.id == current_log_id)
+                )
+                current_log = current_result.scalar_one_or_none()
+                if current_log:
+                    main_log = current_log
+
             if main_log:
                 # 更新已有的 ChatLog：填充 token 数据和消息内容
                 if input_tokens + output_tokens > 0:
@@ -859,25 +869,24 @@ async def _append_voice_log_chunk(
                 content = (user_msg or "").strip()
                 if not content:
                     return
-                if not main_log:
-                    main_log = ChatLog(
-                        user_id=numeric_user_id,
-                        trace_id=session_state.get("trace_id") or f"trace-{user_id}-{uuid.uuid4().hex[:12]}",
-                        conversation_id=conversation_id,
-                        pet_id=int(session_state.get("pet_id") or 0),
-                        bg_id=int(session_state.get("bg_id") or 0),
-                        user_msg=content,
-                        ai_msg="",
-                        tokens_input=0,
-                        tokens_output=0,
-                        duration_ms=0,
-                        mode=mode,
-                        created_at=datetime.now()
-                    )
-                    db.add(main_log)
-                else:
-                    main_log.user_msg = content
-                    main_log.trace_id = main_log.trace_id or session_state.get("trace_id") or main_log.trace_id
+                # 每次新用户消息都创建新的 ChatLog 记录，实现逐轮保存
+                new_log = ChatLog(
+                    user_id=numeric_user_id,
+                    trace_id=session_state.get("trace_id") or f"trace-{user_id}-{uuid.uuid4().hex[:12]}",
+                    conversation_id=conversation_id,
+                    pet_id=int(session_state.get("pet_id") or 0),
+                    bg_id=int(session_state.get("bg_id") or 0),
+                    user_msg=content,
+                    ai_msg="",
+                    tokens_input=0,
+                    tokens_output=0,
+                    duration_ms=0,
+                    mode=mode,
+                    created_at=datetime.now()
+                )
+                db.add(new_log)
+                await db.flush()
+                session_state["current_log_id"] = new_log.id
                 session_state["last_persisted_user_msg"] = content
                 await db.commit()
                 return
@@ -886,6 +895,14 @@ async def _append_voice_log_chunk(
             if not content:
                 return
 
+            # 查找当前轮次的 ChatLog 记录（由 persist_kind="user" 创建）
+            current_log_id = session_state.get("current_log_id")
+            if current_log_id:
+                current_result = await db.execute(
+                    select(ChatLog).where(ChatLog.id == current_log_id)
+                )
+                main_log = current_result.scalar_one_or_none()
+
             if not main_log:
                 main_log = ChatLog(
                     user_id=numeric_user_id,