fix: memory scope_id column overflow and API read mismatch

AgentRuntime writes scope_id as {user_id}:{agent_id} (73 chars) but:
- DB columns were CHAR(36)/VARCHAR(36), causing Data too long errors
- Memory API queried with agent_id only, returning empty results

- Add _mem_scope() helper to build correct scope_id format
- Update all CRUD queries in agent_memory.py to use _mem_scope()
- Enhance login failure logging in auth.py

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

backend/app/api/auth.py

@@ -140,7 +140,9 @@ async def login(
     user = db.query(User).filter(User.username == form_data.username).first()
 
     if not user or not verify_password(form_data.password, user.password_hash):
-        logger.warning(f"登录失败: 用户名 {form_data.username}")
+        logger.warning(f"登录失败: 用户名 {form_data.username}, user_found={user is not None}, pwd_len={len(form_data.password)}")
+        if user:
+            logger.warning(f"  DB hash prefix: {user.password_hash[:30]}...")
         raise UnauthorizedError("用户名或密码错误")
 
     from datetime import timedelta