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/app.ts

@@ -10,6 +10,7 @@ import fastifyMultipart from '@fastify/multipart';
 // Core plugins
 import authPlugin from './core/plugins/auth.plugin';
 import adminGuardPlugin, { setAdminGuardPool } from './core/plugins/admin-guard.plugin';
+import tierGuardPlugin from './core/plugins/tier-guard.plugin';
 import loggingPlugin from './core/plugins/logging.plugin';
 import errorPlugin from './core/plugins/error.plugin';
 import { appConfig } from './core/config/config-loader';
@@ -30,6 +31,7 @@ import { onboardingRoutes } from './features/onboarding';
 import { userPreferencesRoutes } from './features/user-preferences';
 import { userExportRoutes } from './features/user-export';
 import { pool } from './core/config/database';
+import { configRoutes } from './core/config/config.routes';
 
 async function buildApp(): Promise<FastifyInstance> {
   const app = Fastify({
@@ -80,13 +82,16 @@ async function buildApp(): Promise<FastifyInstance> {
   await app.register(adminGuardPlugin);
   setAdminGuardPool(pool);
 
+  // Tier guard plugin - for subscription tier enforcement
+  await app.register(tierGuardPlugin);
+
   // Health check
   app.get('/health', async (_request, reply) => {
     return reply.code(200).send({
       status: 'healthy',
       timestamp: new Date().toISOString(),
       environment: process.env['NODE_ENV'],
-      features: ['admin', 'auth', 'onboarding', 'vehicles', 'documents', 'fuel-logs', 'stations', 'maintenance', 'platform', 'notifications', 'user-profile', 'user-preferences', 'user-export']
+      features: ['admin', 'auth', 'config', 'onboarding', 'vehicles', 'documents', 'fuel-logs', 'stations', 'maintenance', 'platform', 'notifications', 'user-profile', 'user-preferences', 'user-export']
     });
   });
 
@@ -96,7 +101,7 @@ async function buildApp(): Promise<FastifyInstance> {
       status: 'healthy',
       scope: 'api',
       timestamp: new Date().toISOString(),
-      features: ['admin', 'auth', 'onboarding', 'vehicles', 'documents', 'fuel-logs', 'stations', 'maintenance', 'platform', 'notifications', 'user-profile', 'user-preferences', 'user-export']
+      features: ['admin', 'auth', 'config', 'onboarding', 'vehicles', 'documents', 'fuel-logs', 'stations', 'maintenance', 'platform', 'notifications', 'user-profile', 'user-preferences', 'user-export']
     });
   });
 
@@ -136,6 +141,7 @@ async function buildApp(): Promise<FastifyInstance> {
   await app.register(userProfileRoutes, { prefix: '/api' });
   await app.register(userPreferencesRoutes, { prefix: '/api' });
   await app.register(userExportRoutes, { prefix: '/api' });
+  await app.register(configRoutes, { prefix: '/api' });
 
   // 404 handler
   app.setNotFoundHandler(async (_request, reply) => {