fix(web): accept empty chunked API POST requests

Handle untyped zero-byte POST streams at the shared API parser boundary.
Keep request.body undefined and preserve existing typed parsers, 404
fallbacks, authentication and webview proxy stream handling.

Reject nonempty untyped streams on the first byte, and cover parser
boundaries, raw HTTP framing and real WebServer routing with regressions.

test/empty-api-post-server.test.ts

@@ -0,0 +1,50 @@
+/** Verify parser installation through the real server, including API aliases and base paths. */
+import { afterAll, beforeAll, describe, expect, it } from 'vitest';
+import type { FastifyInstance } from 'fastify';
+import { WebServer } from '../src/web/server.js';
+
+describe('empty POST: production server wiring', () => {
+  let server: WebServer;
+  let app: FastifyInstance;
+  beforeAll(async () => {
+    server = new WebServer(0, false, true, '127.0.0.1', undefined, false, '/codeman');
+    await server.start();
+    app = (server as unknown as { app: FastifyInstance }).app;
+  });
+  afterAll(async () => {
+    await server?.stop();
+  });
+
+  it('accepts empty chunked POSTs to a real API through each supported prefix', async () => {
+    for (const prefix of ['/api', '/api/v1', '/codeman/api', '/codeman/api/v1']) {
+      const res = await app.inject({
+        method: 'POST',
+        url: `${prefix}/approvals/session/nonexistent/viewed`,
+        headers: { 'transfer-encoding': 'chunked' },
+      });
+      // The real handler ran and found no session, instead of the parser returning 415.
+      expect(res.statusCode, res.body).toBe(404);
+      expect(res.json().errorCode).toBe('NOT_FOUND');
+    }
+  });
+
+  it('still rejects a nonempty untyped POST before reaching the same handler', async () => {
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/approvals/session/nonexistent/viewed',
+      headers: { 'transfer-encoding': 'chunked' },
+      payload: '{}',
+    });
+    expect(res.statusCode).toBe(415);
+  });
+
+  it('preserves the API 404 envelope for an unknown empty chunked POST', async () => {
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/nonexistent',
+      headers: { 'transfer-encoding': 'chunked' },
+    });
+    expect(res.statusCode).toBe(404);
+    expect(res.json().errorCode).toBe('NOT_FOUND');
+  });
+});