feat(privacy): user blocklist with private chat enforcement

- add blocked_users table and migration
- add users API: block, unblock, list blocked users
- prevent private chat creation and private messaging when block relation exists
- add block/unblock action in private chat info panel

app/chats/service.py

@@ -22,7 +22,7 @@ from app.messages.repository import (
     list_chat_message_ids,
 )
 from app.realtime.presence import get_users_online_map
-from app.users.repository import get_user_by_id
+from app.users.repository import get_user_by_id, has_block_relation_between_users
 
 
 async def serialize_chat_for_user(db: AsyncSession, *, user_id: int, chat: Chat) -> ChatRead:
@@ -103,6 +103,11 @@ async def create_chat_for_user(db: AsyncSession, *, creator_id: int, payload: Ch
             detail="Private chat requires exactly one target user.",
         )
     if payload.type == ChatType.PRIVATE:
+        if await has_block_relation_between_users(db, user_a_id=creator_id, user_b_id=member_ids[0]):
+            raise HTTPException(
+                status_code=status.HTTP_403_FORBIDDEN,
+                detail="Cannot create private chat due to block settings",
+            )
         existing_chat = await repository.find_private_chat_between_users(
             db,
             user_a_id=creator_id,