feat(auth): add TOTP 2FA setup and login verification

- add user twofa fields and migration

- add 2FA setup/enable/disable endpoints

- enforce OTP on login when 2FA enabled

- add web login OTP field and settings UI

app/users/schemas.py

@@ -21,6 +21,7 @@ class UserRead(UserBase):
     bio: str | None = None
     email_verified: bool
     allow_private_messages: bool
+    twofa_enabled: bool = False
     created_at: datetime
     updated_at: datetime