fix(l1): T8 review fixes — oauth status const + bulk-invite structured error

- oauth.py: use status.HTTP_402_PAYMENT_REQUIRED constant (was raw 402)
- accounts.py bulk-invite: catch HTTPException separately to preserve
  structured detail dict in failed-row error (was stringified repr,
  unparseable by clients)
- Add bulk-invite per-row 402 test verifying structured error preserved

T8 code review identified these as Important issues. Functional change is
the bulk-invite fix; clients can now parse seat-limit errors from bulk
responses. 13/13 seat-enforcement tests pass.

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

backend/app/api/endpoints/accounts.py

@@ -395,6 +395,8 @@ async def create_invites_bulk(
                 invite.email_sent_at = datetime.now(timezone.utc)
 
             created.append(invite)
+        except HTTPException as exc:
+            failed.append({"email": invite_data.email, "error": exc.detail})
         except Exception as e:
             failed.append({"email": invite_data.email, "error": str(e)})
 

backend/app/api/endpoints/oauth.py

@@ -3,7 +3,7 @@ import string
 from datetime import datetime, timezone
 from typing import Annotated
 
-from fastapi import APIRouter, Depends, HTTPException
+from fastapi import APIRouter, Depends, HTTPException, status
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -131,7 +131,7 @@ async def _sign_in_or_register(
                     seat_result = await check_seat_available(acct, sub, invite_record.role, db)
                     if not seat_result.available:
                         raise HTTPException(
-                            status_code=402,
+                            status_code=status.HTTP_402_PAYMENT_REQUIRED,
                             detail={
                                 "code": "seat_limit_exceeded",
                                 "role": seat_result.role,