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

internal/api/submissions.go

@@ -202,6 +202,23 @@ func (a *API) handleUploadSubmissionContext(w http.ResponseWriter, r *http.Reque
 // query to another user's uploads. Each row is enriched with its linked build's
 // outcome — this list is the only player-visible outlet for a build result, so
 // a failed build is not invisible to the person who submitted it.
+// SubmissionLimits is what one upload may carry, read before sending it.
+type SubmissionLimits struct {
+	MaxContextBytes int64 `json:"max_context_bytes"`
+}
+
+// handleSubmissionLimits reports the effective per-upload context cap
+// ([registry] context_max_bytes), so the panel can refuse an oversized file
+// before streaming it into the edge's own body limit.
+func (a *API) handleSubmissionLimits(w http.ResponseWriter, r *http.Request) {
+	l, ok := a.Submissions.(interface{ ContextLimit() int64 })
+	if !ok {
+		writeError(w, r, errSubmissionsUnavailable)
+		return
+	}
+	writeJSON(w, http.StatusOK, SubmissionLimits{MaxContextBytes: l.ContextLimit()})
+}
+
 func (a *API) handleMySubmissions(w http.ResponseWriter, r *http.Request) {
 	if a.Submissions == nil {
 		writeError(w, r, errSubmissionsUnavailable)

internal/api/submissions_test.go

@@ -934,3 +934,27 @@ func TestUploadSubmissionContextRateLimited(t *testing.T) {
 		t.Fatalf("retry at the same instant after failure: code = %d, want 200 (%s)", w.Code, w.Body.String())
 	}
 }
+
+type limitedSubmissions struct {
+	*fakeSubmissions
+	limit int64
+}
+
+func (l limitedSubmissions) ContextLimit() int64 { return l.limit }
+
+// The panel reads the per-upload cap before sending a file; a lane that cannot
+// report one answers like any unwired submissions endpoint.
+func TestSubmissionLimitsReportsTheContextCap(t *testing.T) {
+	api := appSubAPI(&fakeSubmissions{})
+	api.Submissions = limitedSubmissions{&fakeSubmissions{}, 99614720}
+	w := do(api.ExternalHandler(), "GET", "/api/v1/me/submissions/limits", "", nil)
+	if w.Code != 200 || strings.TrimSpace(w.Body.String()) != `{"max_context_bytes":99614720}` {
+		t.Fatalf("limits = %d %s", w.Code, w.Body.String())
+	}
+
+	api.Submissions = nil
+	w = do(api.ExternalHandler(), "GET", "/api/v1/me/submissions/limits", "", nil)
+	if w.Code != 503 {
+		t.Fatalf("unwired limits = %d %s", w.Code, w.Body.String())
+	}
+}