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/search-tool.ts

@@ -5,7 +5,10 @@ import { createLogger } from '../utils/logger';
 const logger = createLogger('search-tool');
 
 // Cloudflare AI Gateway를 통해 OpenAI API 호출 (지역 제한 우회)
-const OPENAI_API_URL = 'https://gateway.ai.cloudflare.com/v1/d8e5997eb4040f8b489f09095c0f623c/telegram-bot/openai/chat/completions';
+function getOpenAIUrl(env: Env): string {
+  const base = env.OPENAI_API_BASE || 'https://gateway.ai.cloudflare.com/v1/d8e5997eb4040f8b489f09095c0f623c/telegram-bot/openai';
+  return `${base}/chat/completions`;
+}
 
 export const searchWebTool = {
   type: 'function',
@@ -61,7 +64,7 @@ export async function executeSearchWeb(args: { query: string }, env?: Env): Prom
     if (hasKorean && env?.OPENAI_API_KEY) {
       try {
         const translateRes = await retryWithBackoff(
-          () => fetch(OPENAI_API_URL, {
+          () => fetch(getOpenAIUrl(env), {
             method: 'POST',
             headers: {
               'Content-Type': 'application/json',
@@ -101,7 +104,7 @@ export async function executeSearchWeb(args: { query: string }, env?: Env): Prom
 
     const response = await retryWithBackoff(
       () => fetch(
-        `https://api.search.brave.com/res/v1/web/search?q=${encodeURIComponent(translatedQuery)}&count=5`,
+        `${env.BRAVE_API_BASE || 'https://api.search.brave.com/res/v1'}/web/search?q=${encodeURIComponent(translatedQuery)}&count=5`,
         {
           headers: {
             'Accept': 'application/json',
@@ -141,12 +144,12 @@ export async function executeSearchWeb(args: { query: string }, env?: Env): Prom
   }
 }
 
-export async function executeLookupDocs(args: { library: string; query: string }): Promise<string> {
+export async function executeLookupDocs(args: { library: string; query: string }, env?: Env): Promise<string> {
   const { library, query } = args;
   try {
     // Context7 REST API 직접 호출
     // 1. 라이브러리 검색
-    const searchUrl = `https://context7.com/api/v2/libs/search?libraryName=${encodeURIComponent(library)}&query=${encodeURIComponent(query)}`;
+    const searchUrl = `${env?.CONTEXT7_API_BASE || 'https://context7.com/api/v2'}/libs/search?libraryName=${encodeURIComponent(library)}&query=${encodeURIComponent(query)}`;
     const searchResponse = await retryWithBackoff(
       () => fetch(searchUrl),
       { maxRetries: 3 }
@@ -160,7 +163,7 @@ export async function executeLookupDocs(args: { library: string; query: string }
     const libraryId = searchData.libraries[0].id;
 
     // 2. 문서 조회
-    const docsUrl = `https://context7.com/api/v2/context?libraryId=${encodeURIComponent(libraryId)}&query=${encodeURIComponent(query)}`;
+    const docsUrl = `${env?.CONTEXT7_API_BASE || 'https://context7.com/api/v2'}/context?libraryId=${encodeURIComponent(libraryId)}&query=${encodeURIComponent(query)}`;
     const docsResponse = await retryWithBackoff(
       () => fetch(docsUrl),
       { maxRetries: 3 }