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/schemas.py

@@ -31,3 +31,15 @@ class ChatCreateRequest(BaseModel):
     type: ChatType
     title: str | None = Field(default=None, max_length=255)
     member_ids: list[int] = Field(default_factory=list)
+
+
+class ChatMemberAddRequest(BaseModel):
+    user_id: int
+
+
+class ChatMemberRoleUpdateRequest(BaseModel):
+    role: ChatMemberRole
+
+
+class ChatTitleUpdateRequest(BaseModel):
+    title: str = Field(min_length=1, max_length=255)