feat: 自定义 OpenAI 兼容上游聚合 + 权重/并发调度

- custom provider:把任意 v1 端点当账号(base_url+key),按 model id 路由,
  直连不走代理;支持 images/generations、images/edits、Sora 式异步视频
- 调度器全局权重优先 + 每账号并发感知(上游账号可配并发,其余系统固定)
- 后台:添加/编辑上游、自定义模型表单(每档普通/代理价,留空=不支持)、
  账号权重/并发列与导入权重
- grok-video / nano-banana-2(runway) 写入硬编码目录(开源自带);
  去掉 540p,Adobe 视频最低 720p
- UI:SelectMenu 与输入框等高、测试耗时改用秒

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

backend/internal/bootstrap/app.go

@@ -12,6 +12,7 @@ import (
 	"backend/internal/model"
 	"backend/internal/provider/adobe"
 	"backend/internal/provider/chatgpt"
+	"backend/internal/provider/custom"
 	"backend/internal/provider/grok"
 	"backend/internal/provider/imagine"
 	"backend/internal/provider/krea"
@@ -110,7 +111,8 @@ func NewApp(ctx context.Context) (*App, error) {
 	kreaClient := krea.NewClient("")
 	imagineClient := imagine.NewClient("")
 	grokClient := grok.NewClient("")
-	v1Svc := service.NewV1Service(cfg, modelRepo, userRepo, eventRepo, tokenRepo, siteRepo, adobeClient, chatGPTClient, runwayClient, leonardoClient, kreaClient, imagineClient, grokClient, rustfsClient)
+	customClient := custom.NewClient()
+	v1Svc := service.NewV1Service(cfg, modelRepo, userRepo, eventRepo, tokenRepo, siteRepo, adobeClient, chatGPTClient, runwayClient, leonardoClient, kreaClient, imagineClient, grokClient, customClient, rustfsClient)
 	siteSvc := service.NewSiteService(siteRepo, cfg.AppTitle)
 	showcaseSvc := service.NewShowcaseService(showcaseRepo)
 	adminReadSvc := service.NewAdminReadService(cfg, userRepo, modelRepo, eventRepo, siteRepo, tokenRepo, cdkRepo, rustfsClient)