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/core/config/config.routes.ts

@@ -0,0 +1,18 @@
+/**
+ * @ai-summary Configuration API routes
+ * @ai-context Exposes feature tier configuration for frontend consumption
+ */
+
+import { FastifyPluginAsync } from 'fastify';
+import { getAllFeatureConfigs, TIER_LEVELS } from './feature-tiers';
+
+export const configRoutes: FastifyPluginAsync = async (fastify) => {
+  // GET /api/config/feature-tiers - Get all feature tier configurations
+  // Public endpoint - no auth required (config is not sensitive)
+  fastify.get('/config/feature-tiers', async (_request, reply) => {
+    return reply.code(200).send({
+      tiers: TIER_LEVELS,
+      features: getAllFeatureConfigs(),
+    });
+  });
+};