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>

src/types.ts

@@ -13,6 +13,13 @@ export interface Env {
   DEPOSIT_ADMIN_ID?: string;
   BRAVE_API_KEY?: string;
   DEPOSIT_API_SECRET?: string;
+  OPENAI_API_BASE?: string;
+  NAMECHEAP_API_URL?: string;
+  WHOIS_API_URL?: string;
+  CONTEXT7_API_BASE?: string;
+  BRAVE_API_BASE?: string;
+  WTTR_IN_URL?: string;
+  HOSTING_SITE_URL?: string;
   RATE_LIMIT_KV: KVNamespace;
 }