feat(api): 单次上传上限改为 context_max_bytes 可配，Cloudflare 边缘后默认 95Mi，新增 GET /me/submissions/limits 供面板预检

internal/api/api.go

@@ -610,6 +610,7 @@ func (a *API) externalAPIRoutes() []apiRoute {
 		// session is the correct gate (the admin verdict lives below, behind adminOnly).
 		{Method: "POST", Pattern: "/api/v1/me/submissions", h: a.handleCreateSubmission},
 		{Method: "GET", Pattern: "/api/v1/me/submissions", h: a.handleMySubmissions},
+		{Method: "GET", Pattern: "/api/v1/me/submissions/limits", h: a.handleSubmissionLimits},
 		// The blob upload for a submission the caller owns: the request body is the
 		// raw gzip build context, streamed to the derived, id-namespaced location.
 		// App-tier and owner-scoped (the id must belong to the principal), exactly