feat(store): set trial_ends_at on tenant create

CreateTenant now defaults trial_ends_at to NOW() + 14 days when the
new tenant lands in status='trial'. Demo-kind tenants get
status='demo' (per PLATFORM_ARCHITECTURE.md §5d) and trial_ends_at
stays NULL — those flow through the M13.2 demo-provisioning path.

Both store implementations (Memory + Postgres) updated; tests assert
the 14-day window for customers and the absent end for demo kind.

Unblocks M12.1 (portal trial banner can render a real countdown).

Refs: M4.1 + M12.1

internal/store/postgres.go

@@ -90,9 +90,20 @@ func scanTenant(row pgx.Row) (*Tenant, error) {
 func (p *Postgres) CreateTenant(ctx context.Context, in TenantCreate) (*Tenant, error) {
 	kind := firstNonEmpty(in.Kind, "customer")
 	plan := firstNonEmpty(in.Plan, "starter")
+	// Default status = 'trial'; set trial_ends_at = NOW() + 14 days so the
+	// portal's trial banner has a real countdown to render. Demo tenants
+	// (kind=demo) get status='demo' and no trial_ends_at — that's set by
+	// the M13.2 demo provisioning path.
 	row := p.pool.QueryRow(ctx,
-		`INSERT INTO tenants (slug, name, kind, plan, sales_owner)
-		 VALUES ($1, $2, $3::tenant_kind, $4, NULLIF($5, ''))
+		`INSERT INTO tenants (slug, name, kind, plan, status, sales_owner, trial_ends_at)
+		 VALUES (
+		   $1, $2, $3::tenant_kind, $4,
+		   CASE WHEN $3::tenant_kind = 'demo' THEN 'demo'::tenant_status
+		        ELSE 'trial'::tenant_status END,
+		   NULLIF($5, ''),
+		   CASE WHEN $3::tenant_kind = 'demo' THEN NULL
+		        ELSE NOW() + INTERVAL '14 days' END
+		 )
 		 RETURNING id::text, slug, name, status::text, kind::text, plan,
 		           COALESCE(erp_customer_id,''), COALESCE(stripe_cust_id,''),
 		           trial_ends_at, contract_start, contract_end, COALESCE(sales_owner,''),