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/repositories/regions.ts

@@ -5,11 +5,9 @@
 
 import { BaseRepository } from './base';
 import { Region, RegionInput, RepositoryError, ErrorCodes } from '../types';
-import { createLogger } from '../utils/logger';
 
 export class RegionsRepository extends BaseRepository<Region> {
   protected tableName = 'regions';
-  protected logger = createLogger('[RegionsRepository]');
   protected allowedColumns = [
     'provider_id',
     'region_code',
@@ -104,13 +102,8 @@ export class RegionsRepository extends BaseRepository<Region> {
         );
       });
 
-      const results = await this.executeBatch(statements);
-
       // Count successful operations
-      const successCount = results.reduce(
-        (sum, result) => sum + (result.meta.changes ?? 0),
-        0
-      );
+      const successCount = await this.executeBatchCount(statements);
 
       this.logger.info('Upserted regions', {
         providerId,