feat: Implement user tier-based feature gating system (refs #8)

Add subscription tier system to gate features behind Free/Pro/Enterprise tiers.

Backend:
- Create feature-tiers.ts with FEATURE_TIERS config and utilities
- Add /api/config/feature-tiers endpoint for frontend config fetch
- Create requireTier middleware for route-level tier enforcement
- Add subscriptionTier to request.userContext in auth plugin
- Gate scanForMaintenance in documents controller (Pro+ required)
- Add migration to reset scanForMaintenance for free users

Frontend:
- Create useTierAccess hook for tier checking
- Create UpgradeRequiredDialog component (responsive)
- Gate DocumentForm checkbox with lock icon for free users
- Add SubscriptionTier type to profile.types.ts

Documentation:
- Add TIER-GATING.md with usage guide

Tests: 30 passing (feature-tiers, tier-guard, controller)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/features/documents/migrations/003_reset_scan_for_maintenance_free_users.sql

@@ -0,0 +1,10 @@
+-- Migration: Reset scanForMaintenance for free tier users
+-- This migration is part of the tier-gating feature implementation.
+-- scanForMaintenance is now a Pro feature, so existing free users with it enabled need to be reset.
+
+UPDATE documents d
+SET scan_for_maintenance = false
+FROM user_profiles u
+WHERE d.user_id = u.auth0_sub
+  AND u.subscription_tier = 'free'
+  AND d.scan_for_maintenance = true;