feat: complete subscription billing system phases 6-10

Phase 6 - Database-driven tiers:
- Update subscription_service to query database first with legacy fallback
- Add get_tier_info() db parameter and _get_tier_from_legacy() method

Phase 7 - Platform health integration:
- Add get_subscription_capacity() for theoretical vs actual capacity
- Include subscription capacity in full health report

Phase 8 - Background subscription tasks:
- Add reset_period_counters() for billing period resets
- Add check_trial_expirations() for trial management
- Add sync_stripe_status() for Stripe synchronization
- Add cleanup_stale_subscriptions() for maintenance
- Add capture_capacity_snapshot() for daily metrics

Phase 10 - Capacity planning & forecasting:
- Add CapacitySnapshot model for historical tracking
- Create capacity_forecast_service with growth trends
- Add /subscription-capacity, /trends, /recommendations endpoints
- Add /snapshot endpoint for manual captures

Also includes billing API enhancements from phase 4:
- Add upcoming-invoice, change-tier, addon purchase/cancel endpoints
- Add UsageSummary schema for billing page
- Enhance billing.js with addon management functions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/services/stripe_service.py

@@ -251,6 +251,19 @@ class StripeService:
         logger.info(f"Reactivated Stripe subscription {subscription_id}")
         return subscription
 
+    def cancel_subscription_item(self, subscription_item_id: str) -> None:
+        """
+        Cancel a subscription item (used for add-ons).
+
+        Args:
+            subscription_item_id: Stripe subscription item ID
+        """
+        if not self.is_configured:
+            raise ValueError("Stripe is not configured")
+
+        stripe.SubscriptionItem.delete(subscription_item_id)
+        logger.info(f"Cancelled Stripe subscription item {subscription_item_id}")
+
     # =========================================================================
     # Checkout & Portal
     # =========================================================================
@@ -263,6 +276,8 @@ class StripeService:
         success_url: str,
         cancel_url: str,
         trial_days: int | None = None,
+        quantity: int = 1,
+        metadata: dict | None = None,
     ) -> stripe.checkout.Session:
         """
         Create a Stripe Checkout session for subscription signup.
@@ -274,6 +289,8 @@ class StripeService:
             success_url: URL to redirect on success
             cancel_url: URL to redirect on cancel
             trial_days: Optional trial period
+            quantity: Number of items (default 1)
+            metadata: Additional metadata to store
 
         Returns:
             Stripe Checkout Session object
@@ -311,16 +328,21 @@ class StripeService:
                 subscription.stripe_customer_id = customer_id
                 db.flush()
 
+        # Build metadata
+        session_metadata = {
+            "vendor_id": str(vendor.id),
+            "vendor_code": vendor.vendor_code,
+        }
+        if metadata:
+            session_metadata.update(metadata)
+
         session_data = {
             "customer": customer_id,
-            "line_items": [{"price": price_id, "quantity": 1}],
+            "line_items": [{"price": price_id, "quantity": quantity}],
             "mode": "subscription",
             "success_url": success_url,
             "cancel_url": cancel_url,
-            "metadata": {
-                "vendor_id": str(vendor.id),
-                "vendor_code": vendor.vendor_code,
-            },
+            "metadata": session_metadata,
         }
 
         if trial_days: