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

admin/app/composer.json

@@ -8,6 +8,7 @@
     "require": {
         "php": "^8.3",
         "filament/filament": "^3.2",
+        "laravel/cashier": "^16.0",
         "laravel/framework": "^13.8",
         "laravel/tinker": "^3.0",
         "league/flysystem-aws-s3-v3": "^3.35"