fix: P2 medium priority issues - validation and logging

P2-1: Tool selection fallback optimization
- Return only utility tools when no patterns match
- Reduces token usage by ~80% in fallback cases

P2-2: Minimum deposit amount validation
- Add MIN_DEPOSIT_AMOUNT = 1,000원
- Prevents spam with tiny deposits

P2-3: Standardize logging
- Replace console.log/error with structured logger
- bank-sms-parser.ts and security.ts

P2-4: Nameserver format validation
- Add validateNameservers() function
- Check minimum 2 NS, valid hostname format
- Clear error messages in Korean

P2-5: Optimistic lock error context
- Return specific error for version conflicts
- User-friendly message: "동시 요청으로 처리가 지연됨"

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/tools/domain-tool.ts

@@ -46,6 +46,26 @@ function convertNamecheapDate(date: string): string {
   return `${year}-${month.padStart(2, '0')}-${day.padStart(2, '0')}`;
 }
 
+// 네임서버 형식 검증
+function validateNameservers(nameservers: string[]): string | null {
+  if (!nameservers || nameservers.length < 2) {
+    return '❌ 최소 2개의 네임서버가 필요합니다.';
+  }
+
+  const nsPattern = /^[a-zA-Z0-9]([a-zA-Z0-9-]*[a-zA-Z0-9])?(\.[a-zA-Z0-9]([a-zA-Z0-9-]*[a-zA-Z0-9])?)+$/;
+
+  for (const ns of nameservers) {
+    if (!ns || !ns.trim()) {
+      return '❌ 빈 네임서버가 있습니다.';
+    }
+    if (!nsPattern.test(ns.trim())) {
+      return `❌ 잘못된 네임서버 형식: ${ns}`;
+    }
+  }
+
+  return null; // valid
+}
+
 // KV 캐싱 인터페이스
 interface CachedTLDPrice {
   tld: string;
@@ -519,6 +539,11 @@ async function executeDomainAction(
       if (!domain) return '🚫 도메인을 지정해주세요.';
       if (!nameservers?.length) return '🚫 네임서버를 지정해주세요.';
       if (!allowedDomains.includes(domain)) return `🚫 ${domain}은 관리 권한이 없습니다.`;
+
+      // 네임서버 형식 검증
+      const validationError = validateNameservers(nameservers);
+      if (validationError) return validationError;
+
       const result = await callNamecheapApi('set_nameservers', { domain, nameservers }, allowedDomains, env, telegramUserId, db, userId);
       if (isErrorResult(result)) return `🚫 ${result.error}`;
       return `✅ ${domain} 네임서버 변경 완료\n\n${nameservers.map(ns => `• ${ns}`).join('\n')}`;