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>

go.mod

@@ -71,6 +71,7 @@ require (
 	github.com/spf13/afero v1.14.0 // indirect
 	github.com/spf13/cast v1.10.0 // indirect
 	github.com/spf13/pflag v1.0.10 // indirect
+	github.com/stripe/stripe-go/v81 v81.4.0 // indirect
 	github.com/subosito/gotenv v1.6.0 // indirect
 	github.com/tklauser/go-sysconf v0.3.12 // indirect
 	github.com/tklauser/numcpus v0.6.1 // indirect