fix: concurrency safety and API correctness from code review

B3 (HIGH): sora_generation_service.go - Add panic recovery to parallel
S3 URL fetching goroutines. Without recovery, a panic in GetAccessURL
would skip wg.Done() causing wg.Wait() to hang indefinitely.

B2 (MEDIUM): subscription_service.go:549 - Replace bare goroutine
with safego.Go() for consistent panic recovery pattern. All other async
calls in this file already use safego.

B4 (MEDIUM): admin/sora_handler.go - Change ClearUserStorage response
from 200 no-op to 410 Gone. The per-user storage quota was fully removed;
returning success was misleading to callers.

backend/internal/handler/admin/sora_handler.go

@@ -1,6 +1,7 @@
 package admin
 
 import (
+	"net/http"
 	"strconv"
 
 	"github.com/Wei-Shaw/sub2api/internal/pkg/pagination"
@@ -167,8 +168,9 @@ func (h *SoraHandler) ListGenerations(c *gin.Context) {
 // ClearUserStorage 清除用户的 Sora 存储空间（已弃用）。
 //
 // Deprecated: Per-user storage tracking has been removed.
-// This endpoint now returns a success no-op. It will be removed in a future version.
-// Clients should stop calling this endpoint.
+// This endpoint now returns 410 Gone. Per-user Sora storage quota tracking was
+// fully removed in the Sora storage refactoring. Storage management is now
+// handled at the system-default level via SoraQuotaService.
 //
 // DELETE /api/v1/admin/sora/users/:id/storage
 func (h *SoraHandler) ClearUserStorage(c *gin.Context) {
@@ -178,18 +180,20 @@ func (h *SoraHandler) ClearUserStorage(c *gin.Context) {
 		return
 	}
 
-	// 重置用户的存储使用量
-	// NOTE: Per-user SoraStorageUsedBytes field removed.
-	// Storage clearing now handled at the SoraGenerationService level if needed.
-	_, err = h.userRepo.GetByID(c.Request.Context(), userID)
-	if err != nil {
+	// Verify user exists before responding
+	ctx := c.Request.Context()
+	if _, err := h.userRepo.GetByID(ctx, userID); err != nil {
 		response.ErrorFrom(c, err)
 		return
 	}
 
-	// TODO: Implement storage cleanup via SoraGenerationService
 	c.Header("Deprecation", "true")
 	c.Header("Sunset", "2026-12-31")
-	c.Header("Warning", `299 - "Deprecated API: use SoraGenerationService for storage management"`)
-	response.Success(c, gin.H{"message": "User Sora storage cleared (no-op: per-user tracking removed)", "deprecated": true})
+	c.Header("Warning", `299 - "Gone: per-user storage tracking removed, see SoraQuotaService"`)
+	c.JSON(http.StatusGone, gin.H{
+		"error":     "This endpoint is no longer available",
+		"message":   "Per-user Sora storage quota tracking has been removed. Storage is now managed at system level.",
+		"sunset":    "2026-12-31",
+		"deprecated": true,
+	})
 }