Epic 5 Complete: Billing, Subscriptions, and Admin Features

Resolve all 5 deferred items from Epic 5 adversarial code review:
- Migration 124: Add CASCADE to subscriptionstatus enum drop (prevent orphaned references)
- Stripe rate limiting: In-memory per-user limiter (20 calls/60s) on verify-checkout-session
- Subscription request cooldown: 24h cooldown before resubmitting rejected requests
- Token reset date: Initialize on first subscription activation
- Checkout URL validation: Confirmed HTTPS-only (Stripe always returns HTTPS)

Implement Story 5.4 (Usage Tracking & Rate Limit Enforcement):
- Page quota pre-check at HTTP upload layer
- Extend UserRead schema with token quota fields
- Frontend 402 error handling in document upload
- Quota indicator in dashboard sidebar

Story 5.5 (Admin Seed & Approval Flow):
- Seed admin user migration with default credentials warning
- Subscription approval/rejection routes with admin guard
- 24h rejection cooldown enforcement

Story 5.6 (Admin-Only Model Config):
- Global model config visible across all search spaces
- Per-search-space model configs with user access control
- Superuser CRUD for global configs

Additional fixes from code review:
- PageLimitService: PAST_DUE subscriptions enforce free-tier limits
- TokenQuotaService: PAST_DUE subscriptions enforce free-tier limits
- Config routes: Fixed user_id.is_(None) filter on mutation endpoints
- Stripe webhook: Added guard against silent plan downgrade on unrecognized price_id

All changes formatted with Ruff (Python) and Biome (TypeScript).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

_bmad-output/implementation-artifacts/5-3-stripe-webhook-sync.md

@@ -116,3 +116,7 @@ Webhook endpoint **PHẢI** parse raw body bằng `await request.body()` TRƯỚ
 ### Change Log
 - 2026-04-15: Implement Stripe webhook subscription event handlers — subscription lifecycle, invoice payment reset, checkout activation.
 - 2026-04-15: Code review patches applied — 12 fixes: pages_limit upgrade/downgrade, period_end guards, token reset billing reasons, idempotency plan_id check, UTC date fix, unrecognized price ID warning.
+
+## Review Findings (2026-04-15)
+
+- [x] [Review][Patch] Unrecognized Stripe price ID silently downgrades active subscription to "free" [stripe_routes.py] — **Fixed**: Added early return when plan_id=="free" but subscription status is "active" and price ID was unrecognized