feat: DB DI 전환 + download 하위 호환성 + race condition 수정

- middleware(Auth, Idempotency)를 클로저 팩토리 패턴으로 DI 전환
- database.DB/RDB 전역 변수 제거, ConnectMySQL/Redis 값 반환으로 변경
- download API X-API-Version 헤더 + 하위 호환성 규칙 문서화
- SaveGameData PlayTimeDelta 원자적 UPDATE (race condition 해소)

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

pkg/middleware/auth.go

@@ -8,60 +8,63 @@ import (
 	"strings"
 
 	"a301_server/pkg/apperror"
-	"a301_server/pkg/config"
-	"a301_server/pkg/database"
 	"github.com/gofiber/fiber/v2"
 	"github.com/golang-jwt/jwt/v5"
+	"github.com/redis/go-redis/v9"
 )
 
-func Auth(c *fiber.Ctx) error {
-	header := c.Get("Authorization")
-	if !strings.HasPrefix(header, "Bearer ") {
-		return apperror.ErrUnauthorized
-	}
-	tokenStr := strings.TrimPrefix(header, "Bearer ")
-
-	token, err := jwt.Parse(tokenStr, func(t *jwt.Token) (any, error) {
-		if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok {
-			return nil, fmt.Errorf("unexpected signing method")
+// Auth returns a middleware that validates JWT tokens and checks Redis sessions.
+func Auth(rdb *redis.Client, jwtSecret string) fiber.Handler {
+	secretBytes := []byte(jwtSecret)
+	return func(c *fiber.Ctx) error {
+		header := c.Get("Authorization")
+		if !strings.HasPrefix(header, "Bearer ") {
+			return apperror.ErrUnauthorized
 		}
-		return []byte(config.C.JWTSecret), nil
-	})
-	if err != nil || !token.Valid {
-		return apperror.Unauthorized("유효하지 않은 토큰입니다")
-	}
+		tokenStr := strings.TrimPrefix(header, "Bearer ")
 
-	claims, ok := token.Claims.(jwt.MapClaims)
-	if !ok {
-		return apperror.Unauthorized("유효하지 않은 토큰입니다")
-	}
-	userIDFloat, ok := claims["user_id"].(float64)
-	if !ok {
-		return apperror.Unauthorized("유효하지 않은 토큰입니다")
-	}
-	username, ok := claims["username"].(string)
-	if !ok {
-		return apperror.Unauthorized("유효하지 않은 토큰입니다")
-	}
-	role, ok := claims["role"].(string)
-	if !ok {
-		return apperror.Unauthorized("유효하지 않은 토큰입니다")
-	}
-	userID := uint(userIDFloat)
+		token, err := jwt.Parse(tokenStr, func(t *jwt.Token) (any, error) {
+			if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok {
+				return nil, fmt.Errorf("unexpected signing method")
+			}
+			return secretBytes, nil
+		})
+		if err != nil || !token.Valid {
+			return apperror.Unauthorized("유효하지 않은 토큰입니다")
+		}
 
-	// Redis 세션 확인
-	ctx, cancel := context.WithTimeout(context.Background(), redisTimeout)
-	defer cancel()
-	key := fmt.Sprintf("session:%d", userID)
-	stored, err := database.RDB.Get(ctx, key).Result()
-	if err != nil || stored != tokenStr {
-		return apperror.Unauthorized("만료되었거나 로그아웃된 세션입니다")
-	}
+		claims, ok := token.Claims.(jwt.MapClaims)
+		if !ok {
+			return apperror.Unauthorized("유효하지 않은 토큰입니다")
+		}
+		userIDFloat, ok := claims["user_id"].(float64)
+		if !ok {
+			return apperror.Unauthorized("유효하지 않은 토큰입니다")
+		}
+		username, ok := claims["username"].(string)
+		if !ok {
+			return apperror.Unauthorized("유효하지 않은 토큰입니다")
+		}
+		role, ok := claims["role"].(string)
+		if !ok {
+			return apperror.Unauthorized("유효하지 않은 토큰입니다")
+		}
+		userID := uint(userIDFloat)
 
-	c.Locals("userID", userID)
-	c.Locals("username", username)
-	c.Locals("role", role)
-	return c.Next()
+		// Redis 세션 확인
+		ctx, cancel := context.WithTimeout(context.Background(), redisTimeout)
+		defer cancel()
+		key := fmt.Sprintf("session:%d", userID)
+		stored, err := rdb.Get(ctx, key).Result()
+		if err != nil || stored != tokenStr {
+			return apperror.Unauthorized("만료되었거나 로그아웃된 세션입니다")
+		}
+
+		c.Locals("userID", userID)
+		c.Locals("username", username)
+		c.Locals("role", role)
+		return c.Next()
+	}
 }
 
 func AdminOnly(c *fiber.Ctx) error {
@@ -71,14 +74,16 @@ func AdminOnly(c *fiber.Ctx) error {
 	return c.Next()
 }
 
-// ServerAuth validates X-API-Key header for server-to-server communication.
+// ServerAuth returns a middleware that validates X-API-Key header for server-to-server communication.
 // Uses constant-time comparison to prevent timing attacks.
-func ServerAuth(c *fiber.Ctx) error {
-	key := c.Get("X-API-Key")
-	expected := config.C.InternalAPIKey
-	if key == "" || expected == "" || subtle.ConstantTimeCompare([]byte(key), []byte(expected)) != 1 {
-		log.Printf("ServerAuth 실패: IP=%s, Path=%s", c.IP(), c.Path())
-		return apperror.Unauthorized("유효하지 않은 API 키입니다")
+func ServerAuth(apiKey string) fiber.Handler {
+	expectedBytes := []byte(apiKey)
+	return func(c *fiber.Ctx) error {
+		key := c.Get("X-API-Key")
+		if key == "" || len(expectedBytes) == 0 || subtle.ConstantTimeCompare([]byte(key), expectedBytes) != 1 {
+			log.Printf("ServerAuth 실패: IP=%s, Path=%s", c.IP(), c.Path())
+			return apperror.Unauthorized("유효하지 않은 API 키입니다")
+		}
+		return c.Next()
 	}
-	return c.Next()
 }