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/http/handler/app_settings.go

@@ -1,7 +1,9 @@
 package handler
 
 import (
+	"encoding/base64"
 	"net/http"
+	"strings"
 
 	"backend/internal/service"
 	"github.com/gin-gonic/gin"
@@ -15,6 +17,87 @@ func NewAppSettingsHandler(settings *service.AppSettingsService) *AppSettingsHan
 	return &AppSettingsHandler{settings: settings}
 }
 
+// LogoUpload stores a base64 image as the site logo in RustFS (deleting the old
+// one) and persists site.logo. Called on 保存 — not on file pick.
+func (h *AppSettingsHandler) LogoUpload(c *gin.Context) {
+	var body struct {
+		Data        string `json:"data"` // base64, optionally a "data:...;base64," URL
+		ContentType string `json:"content_type"`
+	}
+	if err := c.ShouldBindJSON(&body); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"detail": "invalid request body"})
+		return
+	}
+	raw := strings.TrimSpace(body.Data)
+	if strings.HasPrefix(raw, "data:") {
+		if i := strings.Index(raw, ","); i >= 0 {
+			if body.ContentType == "" {
+				meta := raw[5:i] // e.g. image/png;base64
+				if j := strings.Index(meta, ";"); j >= 0 {
+					body.ContentType = meta[:j]
+				}
+			}
+			raw = raw[i+1:]
+		}
+	}
+	data, err := base64.StdEncoding.DecodeString(raw)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"detail": "图片解码失败"})
+		return
+	}
+	url, err := h.settings.UploadLogo(c.Request.Context(), data, body.ContentType)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"detail": err.Error()})
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"ok": true, "logo": url})
+}
+
+// AssetUpload stores a public image (homepage 底图 etc.) in RustFS and returns
+// its storage path for the caller to save (e.g. as a showcase card's image).
+func (h *AppSettingsHandler) AssetUpload(c *gin.Context) {
+	var body struct {
+		Data        string `json:"data"`
+		ContentType string `json:"content_type"`
+	}
+	if err := c.ShouldBindJSON(&body); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"detail": "invalid request body"})
+		return
+	}
+	raw := strings.TrimSpace(body.Data)
+	if strings.HasPrefix(raw, "data:") {
+		if i := strings.Index(raw, ","); i >= 0 {
+			if body.ContentType == "" {
+				meta := raw[5:i]
+				if j := strings.Index(meta, ";"); j >= 0 {
+					body.ContentType = meta[:j]
+				}
+			}
+			raw = raw[i+1:]
+		}
+	}
+	data, err := base64.StdEncoding.DecodeString(raw)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"detail": "图片解码失败"})
+		return
+	}
+	path, err := h.settings.UploadAsset(c.Request.Context(), data, body.ContentType)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"detail": err.Error()})
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"ok": true, "path": path})
+}
+
+// LogoDelete removes the uploaded logo and falls back to the built-in default.
+func (h *AppSettingsHandler) LogoDelete(c *gin.Context) {
+	if err := h.settings.RemoveLogo(c.Request.Context()); err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"detail": "failed to remove logo"})
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"ok": true, "logo": ""})
+}
+
 func (h *AppSettingsHandler) RegistrationGet(c *gin.Context) {
 	data, err := h.settings.Registration(c.Request.Context())
 	if err != nil {