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/tools/index.ts

@@ -80,13 +80,13 @@ export async function executeTool(
 ): Promise<string> {
   switch (name) {
     case 'get_weather':
-      return executeWeather(args as { city: string });
+      return executeWeather(args as { city: string }, env);
 
     case 'search_web':
       return executeSearchWeb(args as { query: string }, env);
 
     case 'lookup_docs':
-      return executeLookupDocs(args as { library: string; query: string });
+      return executeLookupDocs(args as { library: string; query: string }, env);
 
     case 'get_current_time':
       return executeGetCurrentTime(args as { timezone?: string });