Phase 5 billing: account-based annual sub + tag SKUs + SMS pool + tag lifecycle

- Schema: plans, orders subscription cols + plan_id + period_started_at,
  billing_events log, users.stripe_id + sms_credits, products.price_aud
  (db/schema.sql + Laravel migration, idempotent)
- Laravel/Cashier: Billable User, checkout (annual sub + one-off SKUs with
  Managed Payments tax_code), webhook controller (signature-verified,
  idempotent, BILLING_ENABLED kill-switch), account tag transitions,
  nightly reconcile, Stripe portal link, PlanResource + billing dashboard
- Go frontend: account-level gating (paid sub required), SMS pool
  (included 50/yr + credits, drawn after included), plan caps replace
  constants, 60s plan cache (credits fresh), 25-tag cap (plan max_tags)
- BillingSeeder: personal plan + 3 SKUs + dev paid orders
- Verified test-mode e2e: subscribe/paid/active/alerts, pool exhaust +
  credits resume, lapsed/suspended, cancelled/closed, recover/active,
  webhook idempotency, 25-cap, one-off SKUs, replacement, kill-switch,
  invalid signature 400

frontend/internal/handlers/handlers.go

@@ -26,11 +26,12 @@ type App struct {
 	Sender   sms.Sender
 	Storage  *storage.Client
 	Notifier *notify.Notifier
+	planCache *planCache
 }
 
 // New returns an App with the given dependencies.
 func New(queries *db.Queries, tpl Templates, sender sms.Sender, store *storage.Client, ntf *notify.Notifier) *App {
-	return &App{Queries: queries, Tpl: tpl, Sender: sender, Storage: store, Notifier: ntf}
+	return &App{Queries: queries, Tpl: tpl, Sender: sender, Storage: store, Notifier: ntf, planCache: newPlanCache()}
 }
 
 // PageData is the root data passed to the base layout.