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/config/config.go

@@ -25,6 +25,7 @@ type Config struct {
 	GoogleAuth GoogleAuthConfig
 	AppleIAP   AppleIAPConfig
 	GoogleIAP  GoogleIAPConfig
+	Stripe     StripeConfig
 	Features   FeatureFlags
 }
 
@@ -104,6 +105,14 @@ type GoogleIAPConfig struct {
 	PackageName        string // Android package name (e.g., com.tt.casera)
 }
 
+// StripeConfig holds Stripe payment configuration
+type StripeConfig struct {
+	SecretKey      string // Stripe secret API key
+	WebhookSecret  string // Stripe webhook endpoint signing secret
+	PriceMonthly   string // Stripe Price ID for monthly Pro subscription
+	PriceYearly    string // Stripe Price ID for yearly Pro subscription
+}
+
 type WorkerConfig struct {
 	// Scheduled job times (UTC)
 	TaskReminderHour    int
@@ -248,6 +257,12 @@ func Load() (*Config, error) {
 			ServiceAccountPath: viper.GetString("GOOGLE_IAP_SERVICE_ACCOUNT_PATH"),
 			PackageName:        viper.GetString("GOOGLE_IAP_PACKAGE_NAME"),
 		},
+		Stripe: StripeConfig{
+			SecretKey:     viper.GetString("STRIPE_SECRET_KEY"),
+			WebhookSecret: viper.GetString("STRIPE_WEBHOOK_SECRET"),
+			PriceMonthly:  viper.GetString("STRIPE_PRICE_MONTHLY"),
+			PriceYearly:   viper.GetString("STRIPE_PRICE_YEARLY"),
+		},
 		Features: FeatureFlags{
 			PushEnabled:             viper.GetBool("FEATURE_PUSH_ENABLED"),
 			EmailEnabled:            viper.GetBool("FEATURE_EMAIL_ENABLED"),