feat(chat): add random public_id and fix users blocked route

- add chats.public_id random identifier with migration 0011
- expose public_id in chat API payloads
- use chat public_id in message search UI label
- fix users router order so /users/blocked no longer conflicts with /users/{user_id}

app/chats/schemas.py

@@ -9,6 +9,7 @@ class ChatRead(BaseModel):
     model_config = ConfigDict(from_attributes=True)
 
     id: int
+    public_id: str
     type: ChatType
     title: str | None = None
     display_title: str | None = None