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/schemas/account.py

@@ -27,7 +27,7 @@ class TransferOwnershipRequest(BaseModel):
 
 class AccountInviteCreate(BaseModel):
     email: str = Field(..., max_length=255)
-    role: str = Field("engineer", pattern="^(engineer|viewer)$")
+    role: str = Field("engineer", pattern="^(engineer|viewer|l1_tech)$")
     expires_in_days: Optional[int] = Field(None, ge=1, le=30)