feat(l1): create l1_walk_sessions table with target-consistency check + RLS

Per-session state for L1 walking a ticket. Supports flow/proposal/adhoc
session kinds; check constraint enforces target-consistency (flow_id set
iff kind=flow; flow_proposal_id set iff kind=proposal; both null iff
kind=adhoc). walked_path + walk_notes JSONB columns track step-by-step
progress; resolved/escalated/abandoned terminal statuses captured.
Account-scoped RLS matches the internal_tickets precedent (FORCE RLS +
tenant_isolation policy with COALESCE/NULLIF guard).

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

backend/app/models/__init__.py

@@ -67,6 +67,7 @@ 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
+from .l1_walk_session import L1WalkSession  # noqa: F401
 
 __all__ = [
     "User",
@@ -148,4 +149,5 @@ __all__ = [
     "SalesLead",
     "StripeEvent",
     "InternalTicket",
+    "L1WalkSession",
 ]