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)})