auth(2fa): block setup after enable to avoid secret reissue

app/auth/service.py

@@ -260,6 +260,8 @@ def get_access_session_info(token: str) -> tuple[str, datetime] | None:
 
 async def setup_twofa(db: AsyncSession, user: User) -> tuple[str, str]:
     if user.twofa_enabled and user.twofa_secret:
+        raise HTTPException(status_code=status.HTTP_400_BAD_REQUEST, detail="2FA is already enabled")
+    if user.twofa_secret:
         secret = user.twofa_secret
     else:
         secret = generate_totp_secret()

docs/api-reference.md

@@ -553,6 +553,8 @@ Response:
 }
 ```
 
+If 2FA is already enabled for the account, returns `400` (`"2FA is already enabled"`).
+
 ### POST `/api/v1/auth/2fa/enable`
 
 Auth required.  

docs/core-checklist-status.md

@@ -38,7 +38,7 @@ Legend:
 29. Archive - `DONE`
 30. Blacklist - `DONE`
 31. Privacy - `PARTIAL` (avatar/last-seen/group-invites + PM policy `everyone|contacts|nobody`; policy behavior covered by integration tests, remaining UX/matrix hardening)
-32. Security - `PARTIAL` (sessions + revoke + 2FA base + access-session visibility; revoke-all now force-disconnects active realtime sessions; UX/TOTP recovery flow ongoing)
+32. Security - `PARTIAL` (sessions + revoke + 2FA base + access-session visibility; revoke-all now force-disconnects active realtime sessions; 2FA setup now blocked after enable to prevent secret re-issuance; UX/TOTP recovery flow ongoing)
 33. Realtime Events - `DONE` (connect/disconnect/send/receive/typing/read/delivered/online/offline + chat/message updates)
 34. Sync - `PARTIAL` (cross-device via backend state + realtime; reconciliation improved for loaded chats/messages, chat-info panel now hot-refreshes on `chat_updated`)
 35. Additional - `PARTIAL` (drafts/link preview partial/autoload media basic)

tests/test_auth_flow.py

@@ -1,6 +1,7 @@
 from sqlalchemy import select
 
 from app.auth.models import EmailVerificationToken
+from app.utils.totp import _totp_code
 
 
 async def test_register_verify_login_and_me(client, db_session):
@@ -115,3 +116,35 @@ async def test_revoke_all_sessions_invalidates_access_and_refresh(client, db_ses
 
     refresh_response = await client.post("/api/v1/auth/refresh", json={"refresh_token": refresh_token})
     assert refresh_response.status_code == 401
+
+
+async def test_twofa_setup_is_blocked_when_already_enabled(client, db_session):
+    payload = {
+        "email": "dana@example.com",
+        "name": "Dana",
+        "username": "dana",
+        "password": "strongpass123",
+    }
+    await client.post("/api/v1/auth/register", json=payload)
+    token_row = await db_session.execute(select(EmailVerificationToken).order_by(EmailVerificationToken.id.desc()))
+    verify_token = token_row.scalar_one().token
+    await client.post("/api/v1/auth/verify-email", json={"token": verify_token})
+
+    login_response = await client.post(
+        "/api/v1/auth/login",
+        json={"email": payload["email"], "password": payload["password"]},
+    )
+    assert login_response.status_code == 200
+    access_token = login_response.json()["access_token"]
+    headers = {"Authorization": f"Bearer {access_token}"}
+
+    setup_response = await client.post("/api/v1/auth/2fa/setup", headers=headers)
+    assert setup_response.status_code == 200
+    secret = setup_response.json()["secret"]
+    code = _totp_code(secret)
+
+    enable_response = await client.post("/api/v1/auth/2fa/enable", headers=headers, json={"code": code})
+    assert enable_response.status_code == 200
+
+    setup_again_response = await client.post("/api/v1/auth/2fa/setup", headers=headers)
+    assert setup_again_response.status_code == 400