refactor: P3 low priority improvements

P3-1: Magic numbers to named constants
- SMS_CONTENT_MAX_LENGTH = 500 (bank-sms-parser.ts)
- DEFAULT_HISTORY_LIMIT = 10 (deposit-agent.ts)
- LOG_RESULT_MAX_LENGTH = 200 (deposit-tool.ts)

P3-2: Debug command admin-only access
- Add DEPOSIT_ADMIN_ID check for /debug command
- Return "관리자만 사용 가능" for non-admins

P3-3: Stats query null handling
- Replace || with ?? for COUNT results
- Prevents 0 being overwritten incorrectly

P3-4: Memory save error logging
- Add logger.debug for saveMemorySilently failures
- Non-critical but helps debug memory issues

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

src/openai-service.ts

@@ -413,7 +413,9 @@ export async function generateOpenAIResponse(
       const saveableInfo = extractSaveableInfo(userMessage);
       if (saveableInfo) {
         // 비동기로 저장, 응답 지연 없음
-        saveMemorySilently(db, telegramUserId, saveableInfo).catch(() => {});
+        saveMemorySilently(db, telegramUserId, saveableInfo).catch(err => {
+          logger.debug('Memory save failed (non-critical)', { error: (err as Error).message });
+        });
       }
 
       return finalResponse;