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>

src/types.ts

@@ -3,6 +3,7 @@ export interface Env {
   AI: Ai;
   BOT_TOKEN: string;
   WEBHOOK_SECRET: string;
+  ENVIRONMENT?: string;
   SUMMARY_THRESHOLD?: string;
   MAX_SUMMARIES_PER_USER?: string;
   N8N_WEBHOOK_URL?: string;