feat(l1): seat_enforcement service for engineer + L1 seat limits

Shared helper used by invite, accept-invite, and role-change endpoints
(integrated in T8). Counts active users by role against role-specific
seat limit on subscription (engineer → seat_limit, l1_tech → l1_seat_limit).
None limit = unlimited.

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

backend/app/schemas/seat_enforcement.py

@@ -0,0 +1,18 @@
+from typing import Literal, Optional
+
+from pydantic import BaseModel
+
+
+Role = Literal['engineer', 'l1_tech']
+
+
+class SeatCheckResult(BaseModel):
+    available: bool
+    current: int
+    limit: Optional[int]   # None = unlimited
+    role: Role
+
+
+class SeatUsage(BaseModel):
+    engineer: SeatCheckResult
+    l1_tech: SeatCheckResult