feat(auth): guard login/password paths against OAuth-only users

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/api/endpoints/auth.py

@@ -62,6 +62,22 @@ def _generate_display_code() -> str:
     return ''.join(secrets.choice(chars) for _ in range(8))
 
 
+async def _reject_if_oauth_only(db: AsyncSession, user) -> None:
+    """If the user has no password_hash, raise 400 with a list of linked
+    providers so the client can redirect them to the right OAuth flow."""
+    if user is None or user.password_hash is not None:
+        return
+    from app.models.oauth_identity import OAuthIdentity
+    result = await db.execute(
+        select(OAuthIdentity.provider).where(OAuthIdentity.user_id == user.id)
+    )
+    providers = [row for row in result.scalars().all()]
+    raise HTTPException(
+        status_code=400,
+        detail={"error": "use_oauth_provider", "providers": providers},
+    )
+
+
 @router.post("/register", response_model=UserResponse, status_code=status.HTTP_201_CREATED)
 @limiter.limit("3/minute")
 async def register(
@@ -243,6 +259,7 @@ async def login(
     result = await db.execute(select(User).where(User.email == form_data.username))
     user = result.scalar_one_or_none()
 
+    await _reject_if_oauth_only(db, user)
     if not user or not verify_password(form_data.password, user.password_hash):
         raise HTTPException(
             status_code=status.HTTP_401_UNAUTHORIZED,
@@ -280,6 +297,7 @@ async def login_json(
     result = await db.execute(select(User).where(User.email == credentials.email))
     user = result.scalar_one_or_none()
 
+    await _reject_if_oauth_only(db, user)
     if not user or not verify_password(credentials.password, user.password_hash):
         raise HTTPException(
             status_code=status.HTTP_401_UNAUTHORIZED,
@@ -445,6 +463,7 @@ async def change_password(
     db: Annotated[AsyncSession, Depends(get_admin_db)]
 ):
     """Change the current user's password."""
+    await _reject_if_oauth_only(db, current_user)
     if not verify_password(data.current_password, current_user.password_hash):
         raise HTTPException(
             status_code=status.HTTP_401_UNAUTHORIZED,
@@ -488,7 +507,7 @@ async def forgot_password(
     result = await db.execute(select(User).where(User.email == data.email))
     user = result.scalar_one_or_none()
 
-    if user:
+    if user and user.password_hash is not None:
         # Create reset token JWT
         raw_token = create_password_reset_token(str(user.id))
         payload = decode_token(raw_token)