refactor: centralize auth middleware and standardize logging

1. API Key Middleware (api.ts)
- Create apiKeyAuth Hono middleware with timing-safe comparison
- Apply to /deposit/balance and /deposit/deduct routes
- Remove duplicate requireApiKey() calls from handlers
- Reduce ~15 lines of duplicated code

2. Logger Standardization (6 files, 27 replacements)
- webhook.ts: 2 console.error → logger
- message-handler.ts: 7 console → logger
- deposit-matcher.ts: 4 console → logger
- n8n-service.ts: 3 console.error → logger
- circuit-breaker.ts: 8 console → logger
- retry.ts: 3 console → logger

Benefits:
- Single point of auth change
- Structured logging with context
- Better observability in production

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

src/n8n-service.ts

@@ -75,7 +75,7 @@ JSON:`;
       };
     }
   } catch (error) {
-    console.error('Intent analysis error:', error);
+    logger.error('Intent analysis error', error as Error);
   }
 
   // 기본값: 일반 대화
@@ -114,7 +114,12 @@ export async function callN8n(
     });
 
     if (!response.ok) {
-      console.error('n8n error:', response.status, await response.text());
+      const errorText = await response.text();
+      logger.error('n8n API error', new Error(`Status ${response.status}: ${errorText}`), {
+        status: response.status,
+        userId,
+        type
+      });
       return { error: `n8n 호출 실패 (${response.status})` };
     }
 
@@ -128,7 +133,7 @@ export async function callN8n(
 
     return parseResult.data;
   } catch (error) {
-    console.error('n8n fetch error:', error);
+    logger.error('n8n fetch error', error as Error, { userId, type });
     return { error: 'n8n 연결 실패' };
   }
 }