feat(l1): create internal_tickets table with RLS

Tenant-scoped fallback ticket model for accounts without PSA integration.
Tracks customer-name, problem-statement, status lifecycle (open/walking/
resolved/escalated), and optional links to flow/proposal/ai_session/
assigned engineer + PSA promotion ID. Account-scoped RLS policy uses
app.current_account_id session setting.

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

backend/app/models/__init__.py

@@ -66,6 +66,7 @@ from .oauth_identity import OAuthIdentity  # noqa: F401
 from .plan_billing import PlanBilling  # noqa: F401
 from .sales_lead import SalesLead  # noqa: F401
 from .stripe_event import StripeEvent  # noqa: F401
+from .internal_ticket import InternalTicket  # noqa: F401
 
 __all__ = [
     "User",
@@ -146,4 +147,5 @@ __all__ = [
     "PlanBilling",
     "SalesLead",
     "StripeEvent",
+    "InternalTicket",
 ]