refactor: code quality improvements (P3)

## Type Safety
- Add zod runtime validation for external API responses
  * Namecheap API responses (domain-register.ts)
  * n8n webhook responses (n8n-service.ts)
  * User request bodies (routes/api.ts)
  * Replaced unsafe type assertions with safeParse()
  * Proper error handling and logging

## Dead Code Removal
- Remove unused callDepositAgent function (127 lines)
  * Legacy Assistants API code no longer needed
  * Now using direct code execution
  * File reduced from 469 → 345 lines (26.4% reduction)

## Configuration Management
- Extract hardcoded URLs to environment variables
  * Added 7 new vars in wrangler.toml:
    OPENAI_API_BASE, NAMECHEAP_API_URL, WHOIS_API_URL,
    CONTEXT7_API_BASE, BRAVE_API_BASE, WTTR_IN_URL, HOSTING_SITE_URL
  * Updated Env interface in types.ts
  * All URLs have fallback to current production values
  * Enables environment-specific configuration (dev/staging/prod)

## Dependencies
- Add zod 4.3.5 for runtime type validation

## Files Modified
- Configuration: wrangler.toml, types.ts, package.json
- Services: 11 TypeScript files with URL/validation updates
- Total: 15 files, +196/-189 lines

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

wrangler.toml

@@ -12,6 +12,15 @@ N8N_WEBHOOK_URL = "https://n8n.anvil.it.com"  # n8n 연동 (선택)
 DOMAIN_OWNER_ID = "821596605"      # 도메인 관리 권한 Telegram ID
 DEPOSIT_ADMIN_ID = "821596605"     # 예치금 관리 권한 Telegram ID
 
+# API Endpoints
+OPENAI_API_BASE = "https://gateway.ai.cloudflare.com/v1/d8e5997eb4040f8b489f09095c0f623c/telegram-bot/openai"
+NAMECHEAP_API_URL = "https://namecheap-api.anvil.it.com"
+WHOIS_API_URL = "https://whois-api-kappa-inoutercoms-projects.vercel.app"
+CONTEXT7_API_BASE = "https://context7.com/api/v2"
+BRAVE_API_BASE = "https://api.search.brave.com/res/v1"
+WTTR_IN_URL = "https://wttr.in"
+HOSTING_SITE_URL = "https://hosting.anvil.it.com"
+
 [[d1_databases]]
 binding = "DB"
 database_name = "telegram-conversations"