fix: 3차 리뷰 LOW — 에러 메시지 일관성, Redis 타임아웃, 입력 검증

- 5개 핸들러 err.Error() → 제네릭 메시지 (Login, Refresh, SSAFY, Ticket, BossRaid)
- Redis context.Background() → WithTimeout 5s (10곳)
- SprintMultiplier 범위 검증 추가
- 방어적 문서화 (SSAFY 충돌, zip bomb, body limit prefix, 로그 주입)

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

internal/auth/handler.go

@@ -1,6 +1,7 @@
 package auth
 
 import (
+	"log"
 	"regexp"
 	"strconv"
 	"strings"
@@ -70,7 +71,8 @@ func (h *Handler) Login(c *fiber.Ctx) error {
 
 	accessToken, refreshToken, user, err := h.svc.Login(req.Username, req.Password)
 	if err != nil {
-		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": err.Error()})
+		log.Printf("Login failed (username=%s): %v", req.Username, err)
+		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "아이디 또는 비밀번호가 올바르지 않습니다"})
 	}
 
 	c.Cookie(&fiber.Cookie{
@@ -106,7 +108,8 @@ func (h *Handler) Refresh(c *fiber.Ctx) error {
 
 	newAccessToken, newRefreshToken, err := h.svc.Refresh(refreshTokenStr)
 	if err != nil {
-		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": err.Error()})
+		log.Printf("Refresh failed: %v", err)
+		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "토큰 갱신에 실패했습니다"})
 	}
 
 	c.Cookie(&fiber.Cookie{
@@ -219,7 +222,8 @@ func (h *Handler) SSAFYCallback(c *fiber.Ctx) error {
 
 	accessToken, refreshToken, user, err := h.svc.SSAFYLogin(req.Code, req.State)
 	if err != nil {
-		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": err.Error()})
+		log.Printf("SSAFY login failed: %v", err)
+		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "SSAFY 로그인에 실패했습니다"})
 	}
 
 	c.Cookie(&fiber.Cookie{
@@ -263,7 +267,8 @@ func (h *Handler) RedeemLaunchTicket(c *fiber.Ctx) error {
 	}
 	token, err := h.svc.RedeemLaunchTicket(req.Ticket)
 	if err != nil {
-		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": err.Error()})
+		log.Printf("RedeemLaunchTicket failed: %v", err)
+		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "유효하지 않거나 만료된 티켓입니다"})
 	}
 	return c.JSON(fiber.Map{"token": token})
 }