Add Stripe billing, free trials, and cross-platform subscription guards

- Stripe integration: add StripeService with checkout sessions, customer
  portal, and webhook handling for subscription lifecycle events.
- Free trials: auto-start configurable trial on first subscription check,
  with admin-controllable duration and enable/disable toggle.
- Cross-platform guard: prevent duplicate subscriptions across iOS, Android,
  and Stripe by checking existing platform before allowing purchase.
- Subscription model: add Stripe fields (customer_id, subscription_id,
  price_id), trial fields (trial_start, trial_end, trial_used), and
  SubscriptionSource/IsTrialActive helpers.
- API: add trial and source fields to status response, update OpenAPI spec.
- Clean up stale migration and audit docs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/handlers/noauth_test.go

@@ -240,7 +240,7 @@ func TestSubscriptionHandler_NoAuth_Returns401(t *testing.T) {
 	contractorRepo := repositories.NewContractorRepository(db)
 	documentRepo := repositories.NewDocumentRepository(db)
 	subscriptionService := services.NewSubscriptionService(subscriptionRepo, residenceRepo, taskRepo, contractorRepo, documentRepo)
-	handler := NewSubscriptionHandler(subscriptionService)
+	handler := NewSubscriptionHandler(subscriptionService, nil)
 	e := testutil.SetupTestRouter()
 
 	// Register routes WITHOUT auth middleware