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/api/v1/admin/platform_health.py

@@ -144,3 +144,71 @@ async def get_capacity_metrics(
     """Get capacity-focused metrics for planning."""
     metrics = platform_health_service.get_capacity_metrics(db)
     return CapacityMetricsResponse(**metrics)
+
+
+@router.get("/subscription-capacity")
+async def get_subscription_capacity(
+    db: Session = Depends(get_db),
+    current_admin: User = Depends(get_current_admin_api),
+):
+    """
+    Get subscription-based capacity metrics.
+
+    Shows theoretical vs actual capacity based on all vendor subscriptions.
+    """
+    return platform_health_service.get_subscription_capacity(db)
+
+
+@router.get("/trends")
+async def get_growth_trends(
+    days: int = 30,
+    db: Session = Depends(get_db),
+    current_admin: User = Depends(get_current_admin_api),
+):
+    """
+    Get growth trends over the specified period.
+
+    Returns growth rates and projections for key metrics.
+    """
+    from app.services.capacity_forecast_service import capacity_forecast_service
+
+    return capacity_forecast_service.get_growth_trends(db, days=days)
+
+
+@router.get("/recommendations")
+async def get_scaling_recommendations(
+    db: Session = Depends(get_db),
+    current_admin: User = Depends(get_current_admin_api),
+):
+    """
+    Get scaling recommendations based on current capacity and growth.
+
+    Returns prioritized list of recommendations.
+    """
+    from app.services.capacity_forecast_service import capacity_forecast_service
+
+    return capacity_forecast_service.get_scaling_recommendations(db)
+
+
+@router.post("/snapshot")
+async def capture_snapshot(
+    db: Session = Depends(get_db),
+    current_admin: User = Depends(get_current_admin_api),
+):
+    """
+    Manually capture a capacity snapshot.
+
+    Normally run automatically by daily background job.
+    """
+    from app.services.capacity_forecast_service import capacity_forecast_service
+
+    snapshot = capacity_forecast_service.capture_daily_snapshot(db)
+    db.commit()
+
+    return {
+        "id": snapshot.id,
+        "snapshot_date": snapshot.snapshot_date.isoformat(),
+        "total_vendors": snapshot.total_vendors,
+        "total_products": snapshot.total_products,
+        "message": "Snapshot captured successfully",
+    }