fix(server): shed only retryable object-journal reads under load (#19176)

Client polling can exhaust the shared object journal queue and reject log persistence. Bound ordinary object-log and trace reads, keep persistence and internal verification queued, and return retryable 503 responses when reads are busy. Leave feedback exports unsent until every log page is available.

Validation: 72 latest-master affected tests and 10 workflow tests pass locally, with the workflow fixture passing strict TypeScript checks. The preceding broader run passed 280 server tests with two opt-in S3 skips. All applicable checks on d0a937768d are green (56 success, three skipped); exact-head Apex review is 5/5 with no unresolved findings. Background persistence still has no fixed queue bound during a prolonged storage outage.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Co-Authored-By: Paperclip <noreply@paperclip.ing>

server/src/__tests__/error-handler.test.ts

@@ -3,6 +3,7 @@ import { beforeEach, describe, expect, it, vi } from "vitest";
 import { HttpError } from "../errors.js";
 import { errorHandler } from "../middleware/error-handler.js";
 import { mcpDiscoveryHttpFailure, retainMcpConnectionFailure } from "../services/mcp-connection-failure.js";
+import { ObjectJournalBusyError } from "../services/object-byte-journal.js";
 
 const recordResponsibleUserDenialOnActiveRunMock = vi.hoisted(() => vi.fn());
 const captureExceptionMock = vi.hoisted(() => vi.fn());
@@ -115,6 +116,23 @@ describe("errorHandler", () => {
     expect(telemetryMocks.trackErrorHandlerCrash).not.toHaveBeenCalled();
   });
 
+  it("answers a shed object-storage read with a retryable 503 without reporting it", () => {
+    const res = { ...makeRes(), set: vi.fn() } as any;
+    const next = vi.fn() as unknown as NextFunction;
+
+    errorHandler(new ObjectJournalBusyError(), makeReq(), res, next);
+
+    expect(res.set).toHaveBeenCalledWith("Retry-After", "2");
+    expect(res.status).toHaveBeenCalledWith(503);
+    expect(res.json).toHaveBeenCalledWith({
+      error: "Object storage is busy; retry shortly",
+      code: "object_journal_io_busy",
+    });
+    expect(res.err).toBeUndefined();
+    expect(captureExceptionMock).not.toHaveBeenCalled();
+    expect(telemetryMocks.trackErrorHandlerCrash).not.toHaveBeenCalled();
+  });
+
   it("only ends the response when headers were already sent during database shutdown", () => {
     const req = { ...makeReq(), app: { locals: { paperclipDatabaseClosing: true } } } as unknown as Request;
     const res = { ...makeRes(), end: vi.fn(), headersSent: true } as any;