fix: security hardening and performance improvements

Security:
- Add token+secret auth to /setup-webhook and /webhook-info endpoints
- Disable /api/test in production environment (ENVIRONMENT=production)

Performance:
- Add retryWithBackoff to weather-tool (maxRetries: 2)
- Add KV caching to executeLookupDocs (1h TTL)

Code Quality:
- Centralize error messages in src/constants/messages.ts
- Update 5 files to use centralized error constants

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

wrangler.toml

@@ -6,6 +6,7 @@ compatibility_date = "2024-01-01"
 binding = "AI"
 
 [vars]
+ENVIRONMENT = "production"          # 환경 설정 (production | development)
 SUMMARY_THRESHOLD = "20"           # 프로필 업데이트 주기 (메시지 수)
 MAX_SUMMARIES_PER_USER = "3"       # 유지할 프로필 버전 수 (슬라이딩 윈도우)
 N8N_WEBHOOK_URL = "https://n8n.anvil.it.com"  # n8n 연동 (선택)