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/openai-service.ts

@@ -8,7 +8,10 @@ import { metrics } from './utils/metrics';
 const logger = createLogger('openai');
 
 // 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`;
+}
 
 // Circuit Breaker 인스턴스 (전역 공유)
 export const openaiCircuitBreaker = new CircuitBreaker({
@@ -42,6 +45,7 @@ interface OpenAIResponse {
 
 // OpenAI API 호출 (retry + circuit breaker 적용)
 async function callOpenAI(
+  env: Env,
   apiKey: string,
   messages: OpenAIMessage[],
   selectedTools?: typeof tools  // undefined = 도구 없음, 배열 = 해당 도구만 사용
@@ -51,7 +55,7 @@ async function callOpenAI(
   try {
     return await retryWithBackoff(
       async () => {
-        const response = await fetch(OPENAI_API_URL, {
+        const response = await fetch(getOpenAIUrl(env), {
           method: 'POST',
           headers: {
             'Content-Type': 'application/json',
@@ -115,7 +119,7 @@ export async function generateOpenAIResponse(
       const selectedTools = selectToolsForMessage(userMessage);
 
       // 첫 번째 호출
-      let response = await callOpenAI(apiKey, messages, selectedTools);
+      let response = await callOpenAI(env, apiKey, messages, selectedTools);
       let assistantMessage = response.choices[0].message;
 
       logger.info('tool_calls', {
@@ -155,7 +159,7 @@ export async function generateOpenAIResponse(
         messages.push(...toolResults);
 
         // 다시 호출 (도구 없이 응답 생성)
-        response = await callOpenAI(apiKey, messages, undefined);
+        response = await callOpenAI(env, apiKey, messages, undefined);
         assistantMessage = response.choices[0].message;
       }
 
@@ -196,6 +200,7 @@ export async function generateProfileWithOpenAI(
     // Circuit Breaker로 실행 감싸기
     return await openaiCircuitBreaker.execute(async () => {
       const response = await callOpenAI(
+        env,
         apiKey,
         [{ role: 'user', content: prompt }],
         undefined  // 도구 없이 호출