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/feature-tiers.ts

@@ -0,0 +1,73 @@
+/**
+ * @ai-summary Feature tier configuration and utilities
+ * @ai-context Defines feature-to-tier mapping for gating premium features
+ */
+
+import { SubscriptionTier } from '../../features/user-profile/domain/user-profile.types';
+
+// Tier hierarchy: higher number = higher access level
+export const TIER_LEVELS: Record<SubscriptionTier, number> = {
+  free: 0,
+  pro: 1,
+  enterprise: 2,
+} as const;
+
+// Feature configuration interface
+export interface FeatureConfig {
+  minTier: SubscriptionTier;
+  name: string;
+  upgradePrompt: string;
+}
+
+// Feature registry - add new gated features here
+export const FEATURE_TIERS: Record<string, FeatureConfig> = {
+  'document.scanMaintenanceSchedule': {
+    minTier: 'pro',
+    name: 'Scan for Maintenance Schedule',
+    upgradePrompt: 'Upgrade to Pro to automatically extract maintenance schedules from your vehicle manuals.',
+  },
+} as const;
+
+/**
+ * Get numeric level for a subscription tier
+ */
+export function getTierLevel(tier: SubscriptionTier): number {
+  return TIER_LEVELS[tier] ?? 0;
+}
+
+/**
+ * Check if a user tier can access a feature
+ * Higher tiers inherit access to all lower tier features
+ */
+export function canAccessFeature(userTier: SubscriptionTier, featureKey: string): boolean {
+  const feature = FEATURE_TIERS[featureKey];
+  if (!feature) {
+    // Unknown features are accessible by all (fail open for unlisted features)
+    return true;
+  }
+  return getTierLevel(userTier) >= getTierLevel(feature.minTier);
+}
+
+/**
+ * Get the minimum required tier for a feature
+ * Returns null if feature is not gated
+ */
+export function getRequiredTier(featureKey: string): SubscriptionTier | null {
+  const feature = FEATURE_TIERS[featureKey];
+  return feature?.minTier ?? null;
+}
+
+/**
+ * Get full feature configuration
+ * Returns undefined if feature is not registered
+ */
+export function getFeatureConfig(featureKey: string): FeatureConfig | undefined {
+  return FEATURE_TIERS[featureKey];
+}
+
+/**
+ * Get all feature configurations (for API endpoint)
+ */
+export function getAllFeatureConfigs(): Record<string, FeatureConfig> {
+  return { ...FEATURE_TIERS };
+}