refactor: add KV cache, env validation, logger types, constants

- Add KV cache abstraction layer (src/services/kv-cache.ts)
- Add Zod-based env validation (src/utils/env-validation.ts)
- Improve logger types: any → unknown for type safety
- Add centralized constants file (src/constants/index.ts)
- Fix security.ts unused import

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

src/security.ts

@@ -138,6 +138,8 @@ export async function validateWebhookRequest(
 }
 
 // Rate Limiting (Cloudflare KV 기반)
+// NOTE: Future migration to KVCache abstraction layer (kv-cache.ts) planned
+// Current implementation kept for backward compatibility
 interface RateLimitData {
   count: number;
   resetAt: number;