feat(chats): add role-based member management APIs

- add owner/admin/member permission checks

- implement member add/remove, role updates, and leave flow

- add chat title update endpoint for manageable chat types

app/chats/repository.py

@@ -1,4 +1,4 @@
-from sqlalchemy import Select, select
+from sqlalchemy import Select, func, select
 from sqlalchemy.orm import aliased
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -19,6 +19,15 @@ async def add_chat_member(db: AsyncSession, *, chat_id: int, user_id: int, role:
     return member
 
 
+async def delete_chat_member(db: AsyncSession, member: ChatMember) -> None:
+    await db.delete(member)
+
+
+async def count_chat_members(db: AsyncSession, *, chat_id: int) -> int:
+    result = await db.execute(select(func.count(ChatMember.id)).where(ChatMember.chat_id == chat_id))
+    return int(result.scalar_one())
+
+
 def _user_chats_query(user_id: int) -> Select[tuple[Chat]]:
     return (
         select(Chat)