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/tests/feature-tiers.test.ts

@@ -0,0 +1,104 @@
+import {
+  TIER_LEVELS,
+  FEATURE_TIERS,
+  getTierLevel,
+  canAccessFeature,
+  getRequiredTier,
+  getFeatureConfig,
+  getAllFeatureConfigs,
+} from '../feature-tiers';
+
+describe('feature-tiers', () => {
+  describe('TIER_LEVELS', () => {
+    it('defines correct tier hierarchy', () => {
+      expect(TIER_LEVELS.free).toBe(0);
+      expect(TIER_LEVELS.pro).toBe(1);
+      expect(TIER_LEVELS.enterprise).toBe(2);
+    });
+
+    it('enterprise > pro > free', () => {
+      expect(TIER_LEVELS.enterprise).toBeGreaterThan(TIER_LEVELS.pro);
+      expect(TIER_LEVELS.pro).toBeGreaterThan(TIER_LEVELS.free);
+    });
+  });
+
+  describe('FEATURE_TIERS', () => {
+    it('includes scanMaintenanceSchedule feature', () => {
+      const feature = FEATURE_TIERS['document.scanMaintenanceSchedule'];
+      expect(feature).toBeDefined();
+      expect(feature.minTier).toBe('pro');
+      expect(feature.name).toBe('Scan for Maintenance Schedule');
+      expect(feature.upgradePrompt).toBeTruthy();
+    });
+  });
+
+  describe('getTierLevel', () => {
+    it('returns correct level for each tier', () => {
+      expect(getTierLevel('free')).toBe(0);
+      expect(getTierLevel('pro')).toBe(1);
+      expect(getTierLevel('enterprise')).toBe(2);
+    });
+
+    it('returns 0 for unknown tier', () => {
+      expect(getTierLevel('unknown' as any)).toBe(0);
+    });
+  });
+
+  describe('canAccessFeature', () => {
+    const featureKey = 'document.scanMaintenanceSchedule';
+
+    it('denies access for free tier to pro feature', () => {
+      expect(canAccessFeature('free', featureKey)).toBe(false);
+    });
+
+    it('allows access for pro tier to pro feature', () => {
+      expect(canAccessFeature('pro', featureKey)).toBe(true);
+    });
+
+    it('allows access for enterprise tier to pro feature (inheritance)', () => {
+      expect(canAccessFeature('enterprise', featureKey)).toBe(true);
+    });
+
+    it('allows access for unknown feature (fail open)', () => {
+      expect(canAccessFeature('free', 'unknown.feature')).toBe(true);
+      expect(canAccessFeature('pro', 'unknown.feature')).toBe(true);
+      expect(canAccessFeature('enterprise', 'unknown.feature')).toBe(true);
+    });
+  });
+
+  describe('getRequiredTier', () => {
+    it('returns required tier for known feature', () => {
+      expect(getRequiredTier('document.scanMaintenanceSchedule')).toBe('pro');
+    });
+
+    it('returns null for unknown feature', () => {
+      expect(getRequiredTier('unknown.feature')).toBeNull();
+    });
+  });
+
+  describe('getFeatureConfig', () => {
+    it('returns full config for known feature', () => {
+      const config = getFeatureConfig('document.scanMaintenanceSchedule');
+      expect(config).toEqual({
+        minTier: 'pro',
+        name: 'Scan for Maintenance Schedule',
+        upgradePrompt: expect.any(String),
+      });
+    });
+
+    it('returns undefined for unknown feature', () => {
+      expect(getFeatureConfig('unknown.feature')).toBeUndefined();
+    });
+  });
+
+  describe('getAllFeatureConfigs', () => {
+    it('returns copy of all feature configs', () => {
+      const configs = getAllFeatureConfigs();
+      expect(configs['document.scanMaintenanceSchedule']).toBeDefined();
+
+      // Verify it's a copy, not the original
+      configs['test'] = { minTier: 'free', name: 'test', upgradePrompt: '' };
+      expect(FEATURE_TIERS['test' as keyof typeof FEATURE_TIERS]).toBeUndefined();
+    });
+  });
+});