fix(csrf): tag browser auto-retry as [CSRF-debug], keep [CSRF] for real probes (DC-058)

Background: every time dashcaddy-api restarts, the first POST from a
dashboard browser tab hits the missing-CSRF-cookie branch. The
status/js/globals.js secureFetch() wrapper catches the 403 and
auto-retries with a fresh token, so the WARN line is misleading noise.

Live evidence (DNS2, 2026-08-18 10:35:32Z container restart):
  [CSRF] Missing CSRF cookie: POST /api/v1/backups/schedule from 100.121.150.22
  [CSRF] Missing CSRF cookie: POST /api/v1/backups/schedule from 100.121.150.22
  [CSRF] Missing CSRF cookie: POST /api/v1/backups/schedule from 172.17.0.1

Fix: if X-CSRF-Token header is ALSO present, tag the log line [CSRF-debug]
(operator can grep it out as expected noise — the secureFetch retry will
self-heal). A request with NEITHER cookie NOR header (curl probe, exploit
scanner, broken client) keeps the [CSRF] tag.

Threat model: forging a header without the cookie just produces a
different 403 (Invalid CSRF token) — the timingSafeEqual check on lines
248-260 of csrf-protection.js is unchanged. This is a log-only fix.

Tests: 4 new in __tests__/csrf-protection.test.js under
'DC-058: browser-auto-retry vs real-probe log tagging'. Full suite
1982/1982 green on DNS2 worktree.

[self-grade=pending]

dashcaddy-api/__tests__/csrf-protection.test.js

@@ -322,6 +322,89 @@ describe('CSRF Protection', () => {
 
       process.env.NODE_ENV = origEnv;
     });
+
+    // DC-058: differentiate "browser auto-retry" from "real probe" by the
+    // presence of the X-CSRF-Token header. The 403 response is identical in
+    // both branches; only the stderr log tag changes.
+    describe('DC-058: browser-auto-retry vs real-probe log tagging', () => {
+      let stderrSpy;
+      let origEnv;
+
+      beforeEach(() => {
+        origEnv = process.env.NODE_ENV;
+        process.env.NODE_ENV = 'production';
+        stderrSpy = jest.spyOn(process.stderr, 'write').mockImplementation(() => true);
+      });
+
+      afterEach(() => {
+        process.env.NODE_ENV = origEnv;
+        stderrSpy.mockRestore();
+      });
+
+      it('tags missing-cookie with [CSRF-debug] when X-CSRF-Token header also present (browser auto-retry)', () => {
+        const { req, res, next } = createMockReqRes({
+          method: 'POST', path: '/api/v1/backups/schedule',
+          headers: { cookie: '', 'x-csrf-token': 'some-signature-attempt' }
+        });
+        csrfValidationMiddleware(req, res, next);
+
+        // 403 response unchanged
+        expect(res.status).toHaveBeenCalledWith(403);
+        expect(res.json).toHaveBeenCalledWith(
+          expect.objectContaining({ error: expect.stringContaining('DC-100') })
+        );
+        // Log tag is [CSRF-debug]
+        expect(stderrSpy).toHaveBeenCalled();
+        const lastWrite = stderrSpy.mock.calls[stderrSpy.mock.calls.length - 1][0];
+        expect(lastWrite).toContain('[CSRF-debug]');
+        expect(lastWrite).toContain('browser auto-retry');
+        expect(lastWrite).not.toMatch(/^\[CSRF\][^-]/); // not bare [CSRF]
+      });
+
+      it('tags missing-cookie with [CSRF] when no X-CSRF-Token header present (real probe)', () => {
+        const { req, res, next } = createMockReqRes({
+          method: 'POST', path: '/api/v1/backups/schedule',
+          headers: { cookie: '' }
+        });
+        csrfValidationMiddleware(req, res, next);
+
+        expect(res.status).toHaveBeenCalledWith(403);
+        expect(stderrSpy).toHaveBeenCalled();
+        const lastWrite = stderrSpy.mock.calls[stderrSpy.mock.calls.length - 1][0];
+        expect(lastWrite).toContain('[CSRF]');
+        expect(lastWrite).not.toContain('[CSRF-debug]');
+        expect(lastWrite).not.toContain('browser auto-retry');
+      });
+
+      it('keeps [CSRF] tag when cookie present but header missing (curl probe with manual cookie)', () => {
+        const nonce = generateToken();
+        const { req, res, next } = createMockReqRes({
+          method: 'POST', path: '/api/v1/backups/schedule',
+          headers: { cookie: `${CSRF_COOKIE_NAME}=${nonce}` }
+        });
+        csrfValidationMiddleware(req, res, next);
+
+        expect(res.status).toHaveBeenCalledWith(403);
+        expect(stderrSpy).toHaveBeenCalled();
+        const lastWrite = stderrSpy.mock.calls[stderrSpy.mock.calls.length - 1][0];
+        expect(lastWrite).toContain('[CSRF]');
+        expect(lastWrite).not.toContain('[CSRF-debug]');
+      });
+
+      it('headerToken is read from x-csrf-token (lowercased by Express) — lowercase header triggers [CSRF-debug]', () => {
+        // Express/Node lowercases all incoming header keys, so production code
+        // only ever sees lowercase. We test the exact code path here.
+        const { req, res, next } = createMockReqRes({
+          method: 'POST', path: '/api/v1/backups/schedule',
+          headers: { cookie: '', 'x-csrf-token': 'some-signature-attempt' }
+        });
+        csrfValidationMiddleware(req, res, next);
+
+        expect(res.status).toHaveBeenCalledWith(403);
+        const lastWrite = stderrSpy.mock.calls[stderrSpy.mock.calls.length - 1][0];
+        expect(lastWrite).toContain('[CSRF-debug]');
+      });
+    });
   });
 
   describe('renewCSRFToken', () => {