feat(l1): enforce seat limits on invite, accept-invite, role-change

For engineer + l1_tech roles, check_seat_available is called at each
mutation point. Returns 402 Payment Required with structured detail
{code: 'seat_limit_exceeded', role, current, limit, upgrade_url} when
seats are full. Grandfathering: existing over-seated accounts keep
existing users; only new mutations are blocked.

Also updates AccountInviteCreate and AccountRoleUpdate schemas to
accept l1_tech as a valid role value.

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

backend/app/api/endpoints/oauth.py

@@ -118,6 +118,29 @@ async def _sign_in_or_register(
 
     if is_new_user:
         if invite_record is not None:
+            # Seat enforcement: re-check at OAuth accept time (race-condition guard).
+            if invite_record.role in ("engineer", "l1_tech"):
+                from app.core.subscriptions import get_account_subscription
+                from app.services.seat_enforcement import check_seat_available
+                sub = await get_account_subscription(invite_record.account_id, db)
+                if sub is not None:
+                    acct_result = await db.execute(
+                        select(Account).where(Account.id == invite_record.account_id)
+                    )
+                    acct = acct_result.scalar_one()
+                    seat_result = await check_seat_available(acct, sub, invite_record.role, db)
+                    if not seat_result.available:
+                        raise HTTPException(
+                            status_code=402,
+                            detail={
+                                "code": "seat_limit_exceeded",
+                                "role": seat_result.role,
+                                "current": seat_result.current,
+                                "limit": seat_result.limit,
+                                "upgrade_url": "/account/billing",
+                            },
+                        )
+
             # Join the invited account directly — no personal account, no
             # trial creation.
             user = User(