feat: add standalone requireTier middleware (refs #138)

Create reusable preHandler middleware for subscription tier gating.
Composable with requireAuth in route preHandler arrays. Returns 403
TIER_REQUIRED with upgrade prompt for insufficient tier, 500 for
unknown feature keys. Includes 9 unit tests covering all acceptance
criteria.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/core/middleware/require-tier.ts

@@ -0,0 +1,64 @@
+/**
+ * @ai-summary Standalone tier guard middleware for route-level feature gating
+ * @ai-context Returns a Fastify preHandler that checks user subscription tier against feature requirements.
+ *   Must be composed AFTER requireAuth in preHandler arrays.
+ */
+
+import { FastifyRequest, FastifyReply } from 'fastify';
+import { canAccessFeature, getFeatureConfig } from '../config/feature-tiers';
+import { logger } from '../logging/logger';
+
+/**
+ * Creates a preHandler middleware that enforces subscription tier requirements.
+ *
+ * Reads the user's tier from request.userContext.subscriptionTier (set by auth middleware).
+ * Must be placed AFTER requireAuth in the preHandler chain.
+ *
+ * Usage:
+ *   fastify.post('/premium-route', {
+ *     preHandler: [requireAuth, requireTier('fuelLog.receiptScan')],
+ *     handler: controller.method
+ *   });
+ *
+ * @param featureKey - Key from FEATURE_TIERS registry (e.g. 'fuelLog.receiptScan')
+ * @returns Fastify preHandler function
+ */
+export function requireTier(featureKey: string) {
+  return async (request: FastifyRequest, reply: FastifyReply): Promise<void> => {
+    // Validate feature key exists in registry
+    const featureConfig = getFeatureConfig(featureKey);
+    if (!featureConfig) {
+      logger.error('requireTier: unknown feature key', { featureKey });
+      return reply.code(500).send({
+        error: 'INTERNAL_ERROR',
+        message: 'Unknown feature configuration',
+      });
+    }
+
+    // Get user tier from userContext (populated by auth middleware)
+    const currentTier = request.userContext?.subscriptionTier || 'free';
+
+    if (!canAccessFeature(currentTier, featureKey)) {
+      logger.warn('requireTier: access denied', {
+        userId: request.userContext?.userId?.substring(0, 8) + '...',
+        currentTier,
+        requiredTier: featureConfig.minTier,
+        featureKey,
+      });
+
+      return reply.code(403).send({
+        error: 'TIER_REQUIRED',
+        requiredTier: featureConfig.minTier,
+        currentTier,
+        featureName: featureConfig.name,
+        upgradePrompt: featureConfig.upgradePrompt,
+      });
+    }
+
+    logger.debug('requireTier: access granted', {
+      userId: request.userContext?.userId?.substring(0, 8) + '...',
+      currentTier,
+      featureKey,
+    });
+  };
+}