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/services/token_quota_service.py

@@ -97,6 +97,15 @@ class TokenQuotaService:
         tokens_used = user.tokens_used_this_month or 0
         token_limit = user.monthly_token_limit or 0
 
+        # PAST_DUE: enforce free-tier token limit to prevent usage without payment
+        if str(getattr(user, "subscription_status", "")).lower() == "past_due":
+            from app.config import config as app_config  # avoid circular import
+
+            free_limit = app_config.PLAN_LIMITS.get("free", {}).get(
+                "monthly_token_limit", 50000
+            )
+            token_limit = min(token_limit, free_limit)
+
         # Strict boundary: >= means at-limit is also exceeded
         if tokens_used + estimated_tokens >= token_limit and token_limit > 0:
             raise TokenQuotaExceededError(