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/admin_subscription_service.py

@@ -177,7 +177,7 @@ class AdminSubscriptionService:
         )
 
         if not result:
-            raise ResourceNotFoundException(f"Subscription for vendor {vendor_id} not found")
+            raise ResourceNotFoundException("Subscription", str(vendor_id))
 
         return result