feat: 并发分组系统 + 品牌/Logo 上传 + 兑换码开关 + 文档分辨率表

并发分组(新功能):
- 新表 concurrency_groups(名称/上限/默认),用户加 concurrency_group_id
- 启动自动建「默认并发」组(上限10、默认),老用户回填、新注册自动绑定
- 并发计数改用 Redis(自愈 sorted-set + TTL + fail-open):
  · 用户并发(画图台 + API key 合计)受其分组上限限制,0=不限制 → 超返回 429
  · 账号级并发也从内存 gate 换成同一套 Redis(6 处调用点)
  · 移除旧的「已有正在生成的任务」单任务锁
- 后台「并发分组」新菜单:增删改、设默认、用户数;默认组不可删(删别的组成员转默认)
- 用户管理:并发列 + 新建/编辑可选分组
- 个人设置页:账户信息卡(用户名/邮箱/角色/余额/并发);/me 暴露 concurrency_group/limit

品牌 / Logo(上传到 RustFS):
- Logo 改成拖拽/点击上传,点保存才上传;替换自动删旧;branding/ 设为公开且被清理任务 pin 住(永不删)
- 有自定义就用:前台左侧 nav + 后台侧栏 + favicon(浏览器标签);没有则默认 V 图标
- 前台页头还原成文字;首页 Hero 子标题用 site.subtitle(默认那句宣传语,设置页预填)
- 邮件验证码标题用站点名;新增 POST/DELETE /settings/logo + POST /settings/asset(首页底图上传)

兑换码开关:
- 系统设置→积分 新增「开启兑换码」(默认开);关闭后后端拒绝兑换、前台隐藏兑换入口(/site 暴露 cdk_redeem_enabled)

文档 / 分辨率:
- 去掉 quality 参数:size(宽x高)同时决定比例 + 分辨率档(长边映射 1K/2K/4K)
- 文档加「分辨率对照表」(14 个比例 × 1K/2K/4K → size 该传的值);guessRatio 与自定义模型 RATIO_OPTS 对齐到 14 个

其它:
- 删模型时同步清掉各上游账号「支持模型」里的该 id
- 首页设置/兑换码弹窗去固定高度滚动条;展示位弹窗浅色主题适配

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/service/concurrency_group.go

@@ -0,0 +1,107 @@
+package service
+
+import (
+	"context"
+	"errors"
+	"strings"
+
+	"backend/internal/model"
+	"backend/internal/repo"
+)
+
+// ConcurrencyGroupService manages the admin-facing concurrency groups: the
+// definitions live in the DB (repo), the live in-flight counts come from Redis.
+type ConcurrencyGroupService struct {
+	repo *repo.ConcurrencyGroupRepository
+	conc *ConcurrencyService
+}
+
+func NewConcurrencyGroupService(r *repo.ConcurrencyGroupRepository, conc *ConcurrencyService) *ConcurrencyGroupService {
+	return &ConcurrencyGroupService{repo: r, conc: conc}
+}
+
+// List returns every group with its bound-user count.
+func (s *ConcurrencyGroupService) List(ctx context.Context) ([]map[string]any, error) {
+	groups, err := s.repo.List(ctx)
+	if err != nil {
+		return nil, err
+	}
+	counts, _ := s.repo.UserCounts(ctx)
+	out := make([]map[string]any, 0, len(groups))
+	for _, g := range groups {
+		out = append(out, map[string]any{
+			"id":              g.ID,
+			"name":            g.Name,
+			"max_concurrency": g.MaxConcurrency,
+			"is_default":      g.IsDefault,
+			"user_count":      counts[g.ID],
+		})
+	}
+	return out, nil
+}
+
+func (s *ConcurrencyGroupService) Create(ctx context.Context, name string, max int) (*model.ConcurrencyGroup, error) {
+	name = strings.TrimSpace(name)
+	if name == "" {
+		return nil, errors.New("名称不能为空")
+	}
+	if max < 0 {
+		max = 0
+	}
+	g := &model.ConcurrencyGroup{
+		ID: "cg-" + randomUpper(10), Name: name, MaxConcurrency: max, IsDefault: false,
+	}
+	if err := s.repo.Create(ctx, g); err != nil {
+		return nil, err
+	}
+	return g, nil
+}
+
+func (s *ConcurrencyGroupService) Update(ctx context.Context, id string, name *string, max *int) (*model.ConcurrencyGroup, error) {
+	patch := map[string]any{}
+	if name != nil {
+		n := strings.TrimSpace(*name)
+		if n == "" {
+			return nil, errors.New("名称不能为空")
+		}
+		patch["name"] = n
+	}
+	if max != nil {
+		m := *max
+		if m < 0 {
+			m = 0
+		}
+		patch["max_concurrency"] = m
+	}
+	if len(patch) == 0 {
+		return s.repo.Get(ctx, id)
+	}
+	return s.repo.Update(ctx, id, patch)
+}
+
+func (s *ConcurrencyGroupService) SetDefault(ctx context.Context, id string) error {
+	if _, err := s.repo.Get(ctx, id); err != nil {
+		return ErrNotFound
+	}
+	return s.repo.SetDefault(ctx, id)
+}
+
+// Delete removes a group (members fall back to the default group). The default
+// group itself can never be deleted.
+func (s *ConcurrencyGroupService) Delete(ctx context.Context, id string) error {
+	def, err := s.repo.GetDefault(ctx)
+	if err != nil || def == nil {
+		return errors.New("缺少默认并发分组")
+	}
+	if id == def.ID {
+		return errors.New("默认并发分组不可删除")
+	}
+	rows, err := s.repo.Delete(ctx, id, def.ID)
+	if err != nil {
+		return err
+	}
+	if rows == 0 {
+		return ErrNotFound
+	}
+	return nil
+}