refactor: comprehensive code review fixes (security, performance, QA)

## Security Improvements
- Fix timing attack in verifyApiKey with fixed 256-byte buffer
- Fix sortOrder SQL injection with whitelist validation
- Fix rate limiting bypass for non-Cloudflare traffic (fail-closed)
- Remove stack trace exposure in error responses
- Add request_id for audit trail (X-Request-ID header)
- Sanitize origin header to prevent log injection
- Add content-length validation for /sync endpoint (10KB limit)
- Replace Math.random() with crypto.randomUUID() for sync IDs
- Expand sensitive data masking patterns (8 → 18)

## Performance Improvements
- Reduce rate limiter KV reads from 3 to 1 per request (66% reduction)
- Increase sync batch size from 100 to 500 (80% fewer batches)
- Fix health check N+1 query with efficient JOINs
- Fix COUNT(*) Cartesian product with COUNT(DISTINCT)
- Implement shared logger cache pattern across repositories
- Add CacheService singleton pattern in recommend.ts
- Add composite index for recommendation queries
- Implement Anvil pricing query batching (100 per chunk)

## QA Improvements
- Add BATCH_SIZE bounds validation (1-1000)
- Add pagination bounds (page >= 1, MAX_OFFSET = 100000)
- Add min/max range consistency validation
- Add DB reference validation for singleton services
- Add type guards for database result validation
- Add timeout mechanism for external API calls (10-60s)
- Use SUPPORTED_PROVIDERS constant instead of hardcoded list

## Removed
- Remove Vault integration (using Wrangler secrets)
- Remove 6-hour pricing cron (daily sync only)

## Configuration
- Add idx_instance_types_specs_filter composite index
- Add CORS Access-Control-Expose-Headers

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/routes/recommend.ts

@@ -33,6 +33,23 @@ interface RecommendRequestBody {
  */
 const SUPPORTED_SCALES: readonly ScaleType[] = ['small', 'medium', 'large'] as const;
 
+/**
+ * Cached CacheService instance for singleton pattern
+ */
+let cachedCacheService: CacheService | null = null;
+
+/**
+ * Get or create CacheService singleton
+ *
+ * @returns CacheService instance with INSTANCES TTL
+ */
+function getCacheService(): CacheService {
+  if (!cachedCacheService) {
+    cachedCacheService = new CacheService(CACHE_TTL.INSTANCES);
+  }
+  return cachedCacheService;
+}
+
 /**
  * Handle POST /recommend endpoint
  *
@@ -157,7 +174,7 @@ export async function handleRecommend(request: Request, env: Env): Promise<Respo
     // 7. Initialize cache service and generate cache key
     logger.info('[Recommend] Validation passed', { stack, scale, budgetMax });
 
-    const cacheService = new CacheService(CACHE_TTL.INSTANCES);
+    const cacheService = getCacheService();
 
     // Generate cache key from sorted stack, scale, and budget
     // Sort stack to ensure consistent cache keys regardless of order
@@ -270,7 +287,8 @@ export async function handleRecommend(request: Request, env: Env): Promise<Respo
         success: false,
         error: {
           code: 'INTERNAL_ERROR',
-          message: error instanceof Error ? error.message : 'Unknown error',
+          message: 'Recommendation request failed. Please try again later.',
+          request_id: crypto.randomUUID(),
           details: {
             duration_ms: duration,
           },