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/repositories/subscription_repo.go

@@ -262,3 +262,59 @@ func (r *SubscriptionRepository) GetPromotionByID(promotionID string) (*models.P
 	}
 	return &promotion, nil
 }
+
+// === Stripe Lookups ===
+
+// FindByStripeCustomerID finds a subscription by Stripe customer ID
+func (r *SubscriptionRepository) FindByStripeCustomerID(customerID string) (*models.UserSubscription, error) {
+	var sub models.UserSubscription
+	err := r.db.Where("stripe_customer_id = ?", customerID).First(&sub).Error
+	if err != nil {
+		return nil, err
+	}
+	return &sub, nil
+}
+
+// FindByStripeSubscriptionID finds a subscription by Stripe subscription ID
+func (r *SubscriptionRepository) FindByStripeSubscriptionID(subscriptionID string) (*models.UserSubscription, error) {
+	var sub models.UserSubscription
+	err := r.db.Where("stripe_subscription_id = ?", subscriptionID).First(&sub).Error
+	if err != nil {
+		return nil, err
+	}
+	return &sub, nil
+}
+
+// UpdateStripeData updates all three Stripe fields (customer, subscription, price) in one call
+func (r *SubscriptionRepository) UpdateStripeData(userID uint, customerID, subscriptionID, priceID string) error {
+	return r.db.Model(&models.UserSubscription{}).Where("user_id = ?", userID).Updates(map[string]interface{}{
+		"stripe_customer_id":     customerID,
+		"stripe_subscription_id": subscriptionID,
+		"stripe_price_id":        priceID,
+	}).Error
+}
+
+// ClearStripeData clears the Stripe subscription and price IDs (customer ID stays for portal access)
+func (r *SubscriptionRepository) ClearStripeData(userID uint) error {
+	return r.db.Model(&models.UserSubscription{}).Where("user_id = ?", userID).Updates(map[string]interface{}{
+		"stripe_subscription_id": nil,
+		"stripe_price_id":        nil,
+	}).Error
+}
+
+// === Trial Management ===
+
+// SetTrialDates sets the trial start, end, and marks trial as used
+func (r *SubscriptionRepository) SetTrialDates(userID uint, trialStart, trialEnd time.Time) error {
+	return r.db.Model(&models.UserSubscription{}).Where("user_id = ?", userID).Updates(map[string]interface{}{
+		"trial_start": trialStart,
+		"trial_end":   trialEnd,
+		"trial_used":  true,
+	}).Error
+}
+
+// UpdateExpiresAt updates the expires_at field for a user's subscription
+func (r *SubscriptionRepository) UpdateExpiresAt(userID uint, expiresAt time.Time) error {
+	return r.db.Model(&models.UserSubscription{}).Where("user_id = ?", userID).
+		Update("expires_at", expiresAt).Error
+}