fix: message ordering and duplicate messages

Backend:
- Composite ORDER BY (created_at DESC, iteration ASC, id ASC) to fix
  non-deterministic ordering when multiple messages share same second
- Composite key cursor pagination to avoid missing messages at boundary

Android:
- Fix message duplication: guard loadMoreHistory from firing before
  initial scroll completes (isAtTop race → fetchOlderMessages →
  cacheMessagesFromApi with different IDs → Room duplicates)
- Add distinctBy(id) at all message assignment points in ViewModel
- cacheMessagesFromApi: skip insert if same ID or same content exists
- Add relative timestamp display on message bubbles

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/agent_chat.py

@@ -604,7 +604,7 @@ async def list_agent_sessions(
     db: Session = Depends(get_db),
 ):
     """获取 Agent 的会话列表，按最近活跃时间排序。"""
-    from sqlalchemy import func as sa_func, desc
+    from sqlalchemy import func as sa_func, desc, or_
 
     # 验证 agent 存在或有权限
     agent = db.query(Agent).filter(Agent.id == agent_id).first()
@@ -666,6 +666,8 @@ async def get_session_messages(
     db: Session = Depends(get_db),
 ):
     """获取会话的消息历史（分页），从旧到新排序。"""
+    from sqlalchemy import or_
+
     # limit 限制
     limit = min(max(limit, 1), 200)
 
@@ -674,16 +676,22 @@ async def get_session_messages(
         ChatMessage.session_id == session_id,
     )
 
-    # 游标分页：before_id 之前的老消息
+    # 游标分页：before_id 之前的老消息（使用复合键避免 created_at 相同导致遗漏）
     if before_id:
         cursor_msg = db.query(ChatMessage).filter(ChatMessage.id == before_id).first()
         if cursor_msg and cursor_msg.created_at:
-            base_q = base_q.filter(ChatMessage.created_at < cursor_msg.created_at)
+            base_q = base_q.filter(
+                or_(
+                    ChatMessage.created_at < cursor_msg.created_at,
+                    (ChatMessage.created_at == cursor_msg.created_at) & (ChatMessage.id < cursor_msg.id),
+                )
+            )
 
     # 取 N+1 条，判断 has_more（按时间降序取最新 N 条，再反转）
+    # 二级排序 iteration + id 避免同一秒内多条消息顺序不确定
     batch = (
         base_q
-        .order_by(ChatMessage.created_at.desc())
+        .order_by(ChatMessage.created_at.desc(), ChatMessage.iteration.asc(), ChatMessage.id.asc())
         .limit(limit + 1)
         .all()
     )