fix(12.2): close code-review blockers on uploads, JSON caps, and pivot fill

Keep form save behind in-flight uploads, make retries idempotent via X-Upload-Id, cap remaining JSON bodies, and surface pending pivot type errors instead of zeroing them.

Co-authored-by: Cursor <cursoragent@cursor.com>

admin/tests/relation/RelationManager.test.ts

@@ -259,6 +259,8 @@ describe('relation manager row actions and deferral (Phase 12.2, D-03, D-12, D-1
       markDirty: vi.fn(),
       pendingChanges: ref(0),
       revision: ref(0),
+      beginUpload: () => () => {},
+      activeUploads: ref(0),
     }
   }