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>

surfsense_backend/app/schemas/users.py

@@ -6,6 +6,10 @@ from fastapi_users import schemas
 class UserRead(schemas.BaseUser[uuid.UUID]):
     pages_limit: int
     pages_used: int
+    monthly_token_limit: int
+    tokens_used_this_month: int
+    plan_id: str
+    subscription_status: str
     display_name: str | None = None
     avatar_url: str | None = None