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

docs/openapi.yaml

@@ -5278,6 +5278,31 @@ paths:
         '503':
           $ref: '#/components/responses/ServiceUnavailable'
 
+  /api/v1/me/submissions/limits:
+    get:
+      tags: [submissions]
+      operationId: submissionLimits
+      x-felis-face: [external]
+      x-felis-tier: app
+      security: [{ sessionCookie: [] }]
+      summary: The per-upload build-context cap
+      description: >-
+        The effective [registry] context_max_bytes: 1 GiB by default, 95 MiB
+        behind the Cloudflare edge (its proxy refuses bodies over 100 MB before
+        they reach the API). The panel checks a file against it before upload.
+      responses:
+        '200':
+          description: The cap.
+          content:
+            application/json:
+              schema:
+                type: object
+                additionalProperties: false
+                required: [max_context_bytes]
+                properties:
+                  max_context_bytes: {type: integer, format: int64}
+        '503':
+          $ref: '#/components/responses/ServiceUnavailable'
   /api/v1/me/submissions/{id}/context:
     post:
       tags: [submissions]
@@ -5290,7 +5315,11 @@ paths:
         principal; a submission the caller does not own is reported as 404, so
         this endpoint cannot upload to or probe another user's submission. Only a
         pending_review submission accepts a context (409 otherwise); a wrong-format
-        or oversize body is rejected with 400, and an upload that would push the
+        or oversize body is rejected with 400 (the per-upload cap is [registry]
+        context_max_bytes: 1 GiB by default and 95 MiB behind the Cloudflare
+        edge, whose proxy refuses bodies over 100 MB with its own HTML 413
+        before they reach the API; GET /api/v1/me/submissions/limits reports
+        the effective cap so a client can check a file before sending it), and an upload that would push the
         caller past their per-user stored-context budget is refused with 403
         before the excess is persisted. Returns 503 when the deployment's context
         store has no implemented upload transport.