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/domain-register.ts

@@ -1,8 +1,25 @@
+import { z } from 'zod';
 import { Env } from './types';
 import { createLogger } from './utils/logger';
 
 const logger = createLogger('domain-register');
 
+// Zod schemas for API response validation
+const NamecheapRegisterResponseSchema = z.object({
+  registered: z.boolean().optional(),
+  domain: z.string().optional(),
+  error: z.string().optional(),
+  detail: z.string().optional(),
+});
+
+const DomainInfoResponseSchema = z.object({
+  expires: z.string().optional(),
+});
+
+const NameserverResponseSchema = z.object({
+  nameservers: z.array(z.string()).optional(),
+});
+
 interface RegisterResult {
   success: boolean;
   domain?: string;
@@ -22,7 +39,7 @@ export async function executeDomainRegister(
   price: number
 ): Promise<RegisterResult> {
   const apiKey = env.NAMECHEAP_API_KEY;
-  const apiUrl = 'https://namecheap-api.anvil.it.com';
+  const apiUrl = env.NAMECHEAP_API_URL || 'https://namecheap-api.anvil.it.com';
 
   if (!apiKey) {
     return { success: false, error: 'API 키가 설정되지 않았습니다.' };
@@ -58,12 +75,15 @@ export async function executeDomainRegister(
       }),
     });
 
-    const registerResult = await registerResponse.json() as {
-      registered?: boolean;
-      domain?: string;
-      error?: string;
-      detail?: string;
-    };
+    const jsonData = await registerResponse.json();
+    const parseResult = NamecheapRegisterResponseSchema.safeParse(jsonData);
+
+    if (!parseResult.success) {
+      logger.error('Namecheap register response schema validation failed', parseResult.error);
+      return { success: false, error: '도메인 등록 응답 형식이 올바르지 않습니다.' };
+    }
+
+    const registerResult = parseResult.data;
 
     if (!registerResponse.ok || !registerResult.registered) {
       const errorMsg = registerResult.error || registerResult.detail || '도메인 등록에 실패했습니다.';
@@ -112,11 +132,18 @@ export async function executeDomainRegister(
         headers: { 'X-API-Key': apiKey }
       });
       if (infoResponse.ok) {
-        const infoResult = await infoResponse.json() as { expires?: string };
-        if (infoResult.expires) {
-          // MM/DD/YYYY → YYYY-MM-DD 변환
-          const [month, day, year] = infoResult.expires.split('/');
-          expiresAt = `${year}-${month.padStart(2, '0')}-${day.padStart(2, '0')}`;
+        const infoJsonData = await infoResponse.json();
+        const infoParseResult = DomainInfoResponseSchema.safeParse(infoJsonData);
+
+        if (!infoParseResult.success) {
+          logger.warn('Domain info response schema validation failed', { domain });
+        } else {
+          const infoResult = infoParseResult.data;
+          if (infoResult.expires) {
+            // MM/DD/YYYY → YYYY-MM-DD 변환
+            const [month, day, year] = infoResult.expires.split('/');
+            expiresAt = `${year}-${month.padStart(2, '0')}-${day.padStart(2, '0')}`;
+          }
         }
       }
 
@@ -125,8 +152,15 @@ export async function executeDomainRegister(
         headers: { 'X-API-Key': apiKey }
       });
       if (nsResponse.ok) {
-        const nsResult = await nsResponse.json() as { nameservers?: string[] };
-        nameservers = nsResult.nameservers || [];
+        const nsJsonData = await nsResponse.json();
+        const nsParseResult = NameserverResponseSchema.safeParse(nsJsonData);
+
+        if (!nsParseResult.success) {
+          logger.warn('Nameserver response schema validation failed', { domain });
+        } else {
+          const nsResult = nsParseResult.data;
+          nameservers = nsResult.nameservers || [];
+        }
       }
     } catch (infoError) {
       console.log(`[DomainRegister] 도메인 정보 조회 실패 (무시):`, infoError);