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', () => {

dashcaddy-api/src/security/csrf-protection.js

@@ -214,14 +214,30 @@ function csrfValidationMiddleware(req, res, next) {
     return next();
   }
 
-  // Validate both values exist
+  // DC-058: differentiate "browser auto-retry" from "real probe" using the
+  // X-CSRF-Token header as a signal. The dashboard JS in status/js/globals.js
+  // secureFetch() pre-fetches /api/v1/csrf-token (which sets the CSRF cookie
+  // via csrfCookieMiddleware) before posting; if the GET raced with container
+  // restart OR the user cleared cookies mid-session, the POST can arrive with
+  // a header but no cookie. secureFetch catches the 403 and auto-retries
+  // with a fresh token (lines 225-238 of globals.js). For these "has header
+  // but no cookie" misses, tag the log line [CSRF-debug] — operators can
+  // grep them out as expected noise. A request with NEITHER cookie NOR
+  // header (curl probe, exploit scanner, broken client) keeps the louder
+  // [CSRF] tag.
   if (!cookieNonce) {
-    process.stderr.write(`[CSRF] Missing CSRF cookie: ${method} ${req.path} from ${req.ip}\n`);
+    const isLikelyBrowserAutoRetry = !!headerToken;
+    const tag = isLikelyBrowserAutoRetry ? '[CSRF-debug]' : '[CSRF]';
+    process.stderr.write(`${tag} Missing CSRF cookie: ${method} ${req.path} from ${req.ip}` +
+      (isLikelyBrowserAutoRetry ? ' (browser auto-retry — header present, expect self-heal)' : '') + '\n');
     return errorResponse(res, 403, '[DC-100] CSRF token missing', {
       message: 'CSRF cookie not found. Please refresh the page (Ctrl+Shift+R) and try again.'
     });
   }
 
+  // Cookie present but no header — a real browser POST always sends both, so
+  // header-less is suspicious (curl probe with manual cookie, misconfigured
+  // client). Keep WARN level.
   if (!headerToken) {
     process.stderr.write(`[CSRF] Missing CSRF header: ${method} ${req.path} from ${req.ip}\n`);
     return errorResponse(res, 403, '[DC-100] CSRF token missing', {