feat: 보안 수정 + Prometheus 메트릭 + 단위 테스트 추가

보안:
- Zip Bomb 방어 (io.LimitReader 100MB)
- Redis Del 에러 로깅 (auth, idempotency)
- 로그인 실패 로그에서 username 제거
- os.Remove 에러 로깅

모니터링:
- Prometheus 메트릭 미들웨어 + /metrics 엔드포인트
- http_requests_total, http_request_duration_seconds 등 4개 메트릭

테스트:
- download (11), chain (10), bossraid (20) = 41개 단위 테스트

기타:
- DB 모델 GORM 인덱스 태그 추가
- launcherHash 필드 + hashFileToHex() 추가

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/middleware/idempotency.go

@@ -85,7 +85,9 @@ func Idempotency(c *fiber.Ctx) error {
 		// Processing failed — remove the key so it can be retried
 		delCtx, delCancel := context.WithTimeout(context.Background(), redisTimeout)
 		defer delCancel()
-		database.RDB.Del(delCtx, redisKey)
+		if delErr := database.RDB.Del(delCtx, redisKey).Err(); delErr != nil {
+			log.Printf("WARNING: idempotency cache delete failed (key=%s): %v", key, delErr)
+		}
 		return err
 	}
 
@@ -104,7 +106,9 @@ func Idempotency(c *fiber.Ctx) error {
 		// Non-success — allow retry by removing the key
 		delCtx, delCancel := context.WithTimeout(context.Background(), redisTimeout)
 		defer delCancel()
-		database.RDB.Del(delCtx, redisKey)
+		if delErr := database.RDB.Del(delCtx, redisKey).Err(); delErr != nil {
+			log.Printf("WARNING: idempotency cache delete failed (key=%s): %v", key, delErr)
+		}
 	}
 
 	return nil

pkg/middleware/metrics.go

@@ -0,0 +1,25 @@
+package middleware
+
+import (
+	"strconv"
+	"time"
+
+	"a301_server/pkg/metrics"
+
+	"github.com/gofiber/fiber/v2"
+)
+
+// Metrics records HTTP request count and duration as Prometheus metrics.
+func Metrics(c *fiber.Ctx) error {
+	start := time.Now()
+	err := c.Next()
+	duration := time.Since(start).Seconds()
+
+	status := strconv.Itoa(c.Response().StatusCode())
+	path := c.Route().Path // use route pattern to avoid cardinality explosion
+	method := c.Method()
+
+	metrics.HTTPRequestsTotal.WithLabelValues(method, path, status).Inc()
+	metrics.HTTPRequestDuration.WithLabelValues(method, path).Observe(duration)
+	return err
+}