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/n8n-service.ts

@@ -1,4 +1,14 @@
+import { z } from 'zod';
 import { Env, IntentAnalysis, N8nResponse } from './types';
+import { createLogger } from './utils/logger';
+
+const logger = createLogger('n8n-service');
+
+// Zod schema for N8n webhook response validation
+const N8nResponseSchema = z.object({
+  reply: z.string().optional(),
+  error: z.string().optional(),
+});
 
 // n8n으로 처리할 기능 목록 (참고용)
 // - weather: 날씨
@@ -104,8 +114,15 @@ export async function callN8n(
       return { error: `n8n 호출 실패 (${response.status})` };
     }
 
-    const data = await response.json() as N8nResponse;
-    return data;
+    const jsonData = await response.json();
+    const parseResult = N8nResponseSchema.safeParse(jsonData);
+
+    if (!parseResult.success) {
+      logger.error('N8n response schema validation failed', parseResult.error);
+      return { error: 'n8n 응답 형식 오류' };
+    }
+
+    return parseResult.data;
   } catch (error) {
     console.error('n8n fetch error:', error);
     return { error: 'n8n 연결 실패' };