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/service.py

@@ -66,6 +66,7 @@ async def serialize_chat_for_user(db: AsyncSession, *, user_id: int, chat: Chat)
     return ChatRead.model_validate(
         {
             "id": chat.id,
+            "public_id": chat.public_id,
             "type": chat.type,
             "title": chat.title,
             "display_title": display_title,
@@ -375,6 +376,7 @@ async def discover_public_chats_for_user(db: AsyncSession, *, user_id: int, quer
         ChatDiscoverRead.model_validate(
             {
                 "id": chat.id,
+                "public_id": chat.public_id,
                 "type": chat.type,
                 "title": chat.title,
                 "handle": chat.handle,