feat(image-storage): 异步生图对象存储改为后台配置，保存即生效

此前开启异步生图必须改服务器上的 config.yaml 并重启容器（#4542），且若想
复用已配置的备份 S3，还得把同一套凭证再填一遍（#4458）。

- 新增 ImageStorageSettingService：配置存 settings 表，SecretAccessKey 经
  SecretEncryptor 加密落库、读回脱敏、留空表示沿用旧值，与备份 S3 配置同一套做法。
- reuse_backup_s3（默认开）直接借用 backup_s3_config 的端点与密钥，只用自己的
  bucket/prefix 区分对象，因此备份走 backups/、图片走 images/，且密钥不会在库里存两份。
- ImageTaskService 的启用状态改由 ImageStorageResolver 在运行时解析并缓存，
  保存设置后 Invalidate 使下次请求重建客户端——不再需要重启。
- repository 侧由提供实例改为提供工厂，客户端才可能在运行期重建。
- 轮询接口的门控从 enabled() 放宽为 Pollable()：关掉开关只拒绝新提交，
  已受理的任务仍可取回结果，不再被中途吞掉。
- config.yaml 的 image_storage 保留为回落，后台从未保存过时沿用，
  升级前已用配置文件开启的部署不受影响。
- 管理端 GET/PUT/POST /admin/backups/image-storage，PUT 与备份 S3 配置一样要求
  step-up 2FA：改写存储目标同样能把生成内容导向外部账号。

注：go generate ./cmd/server 在当前 upstream 基线上即失败（securityaudit.
PromptAdminService 缺 provider），故 wire_gen.go 为手工同步。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VHreE5pzCkSYz7J45fmd2Y

backend/internal/handler/image_task_admin_toggle_test.go

@@ -0,0 +1,134 @@
+//go:build unit
+
+package handler
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	middleware2 "github.com/Wei-Shaw/sub2api/internal/server/middleware"
+	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
+)
+
+type toggleSettingRepo struct {
+	mu     sync.Mutex
+	values map[string]string
+}
+
+func (r *toggleSettingRepo) Get(context.Context, string) (*service.Setting, error) { return nil, nil }
+func (r *toggleSettingRepo) GetValue(_ context.Context, key string) (string, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	return r.values[key], nil
+}
+
+func (r *toggleSettingRepo) Set(_ context.Context, key, value string) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.values[key] = value
+	return nil
+}
+
+func (r *toggleSettingRepo) GetMultiple(context.Context, []string) (map[string]string, error) {
+	return map[string]string{}, nil
+}
+func (r *toggleSettingRepo) SetMultiple(context.Context, map[string]string) error { return nil }
+func (r *toggleSettingRepo) GetAll(context.Context) (map[string]string, error) {
+	return map[string]string{}, nil
+}
+func (r *toggleSettingRepo) Delete(context.Context, string) error { return nil }
+
+type passthroughEncryptor struct{}
+
+func (passthroughEncryptor) Encrypt(plaintext string) (string, error)  { return plaintext, nil }
+func (passthroughEncryptor) Decrypt(ciphertext string) (string, error) { return ciphertext, nil }
+
+type noopImageStorage struct{}
+
+func (noopImageStorage) Save(context.Context, string, string, []byte) (string, error) {
+	return "https://cdn.example.test/object.png", nil
+}
+
+// TestAsyncImageEnablesWithoutRestart drives the actual HTTP path for the bug behind
+// #4458 and #4542: with object storage unconfigured the async endpoint 404s, and the
+// only way to turn it on used to be editing config.yaml and restarting the container.
+// Flipping the admin setting must flip the endpoint over in the same process.
+func TestAsyncImageEnablesWithoutRestart(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	repo := &toggleSettingRepo{values: map[string]string{}}
+	backup := service.NewBackupService(repo, &config.Config{}, passthroughEncryptor{}, nil, nil)
+	factory := func(context.Context, *config.ImageStorageConfig) (service.ImageStorage, error) {
+		return noopImageStorage{}, nil
+	}
+	settings := service.NewImageStorageSettingService(repo, passthroughEncryptor{}, backup, factory, config.ImageStorageConfig{})
+
+	store := &asyncImageMemoryStore{tasks: make(map[string]*service.ImageTaskRecord)}
+	tasks := service.NewImageTaskServiceWithResolver(store, settings.Resolver(), time.Hour, time.Minute)
+
+	h := &AsyncImageHandler{tasks: tasks}
+	h.execute = func(_ string, c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"created": 1, "data": []gin.H{{"url": "https://upstream.test/i.png"}}})
+	}
+
+	router := gin.New()
+	router.Use(func(c *gin.Context) {
+		groupID := int64(3)
+		c.Set(string(middleware2.ContextKeyAPIKey), &service.APIKey{
+			ID: 9, UserID: 7, GroupID: &groupID,
+			Group: &service.Group{ID: groupID, Platform: service.PlatformOpenAI, AllowImageGeneration: true},
+		})
+		c.Next()
+	})
+	router.POST("/v1/images/generations/async", h.Submit)
+	router.GET("/v1/images/tasks/:task_id", h.Get)
+
+	submit := func() *httptest.ResponseRecorder {
+		req := httptest.NewRequest(http.MethodPost, "/v1/images/generations/async",
+			strings.NewReader(`{"model":"gpt-image-1","prompt":"a lighthouse"}`))
+		req.Header.Set("Content-Type", "application/json")
+		rec := httptest.NewRecorder()
+		router.ServeHTTP(rec, req)
+		return rec
+	}
+
+	rec := submit()
+	require.Equal(t, http.StatusNotFound, rec.Code, "disabled until an admin configures object storage")
+	require.Contains(t, rec.Body.String(), "async image tasks are not enabled")
+
+	// The admin saves the setting — no restart, same process.
+	_, err := settings.Update(context.Background(), service.ImageStorageSettings{
+		Enabled: true, Bucket: "my-images",
+		Endpoint: "https://acct.r2.cloudflarestorage.com", AccessKeyID: "ak", SecretAccessKey: "sk",
+	})
+	require.NoError(t, err)
+
+	rec = submit()
+	require.Equal(t, http.StatusAccepted, rec.Code, "the endpoint must go live as soon as the setting is saved")
+
+	var accepted struct {
+		TaskID  string `json:"task_id"`
+		PollURL string `json:"poll_url"`
+	}
+	require.NoError(t, json.Unmarshal(rec.Body.Bytes(), &accepted))
+	require.NotEmpty(t, accepted.TaskID)
+
+	// Turning the feature back off must not strand a task that was already accepted.
+	_, err = settings.Update(context.Background(), service.ImageStorageSettings{Enabled: false})
+	require.NoError(t, err)
+
+	require.Equal(t, http.StatusNotFound, submit().Code, "new submissions are refused again")
+
+	pollRec := httptest.NewRecorder()
+	router.ServeHTTP(pollRec, httptest.NewRequest(http.MethodGet, accepted.PollURL, nil))
+	require.Equal(t, http.StatusOK, pollRec.Code, "an already-accepted task stays pollable after the switch is turned off")
+}