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/utils/id_generator.py

@@ -0,0 +1,9 @@
+import secrets
+import string
+
+
+_ALPHABET = string.ascii_letters + string.digits
+
+
+def generate_public_id(length: int = 12) -> str:
+    return "".join(secrets.choice(_ALPHABET) for _ in range(length))