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>

frontend/src/features/settings/types/profile.types.ts

@@ -2,12 +2,15 @@
  * @ai-summary User profile types for settings feature
  */
 
+export type SubscriptionTier = 'free' | 'pro' | 'enterprise';
+
 export interface UserProfile {
   id: string;
   auth0Sub: string;
   email: string;
   displayName?: string;
   notificationEmail?: string;
+  subscriptionTier: SubscriptionTier;
   createdAt: string;
   updatedAt: string;
 }