audit 4: sign-in timing and sessions, server timeouts, polling that recovers

Steps 1-3 of stage 4, tested together (their docs share files).

Step 1 (M8, L15, L16): an unknown name pays a bcrypt comparison at the
panel's work factor too (noSuchUserHash, made once) -
TestLoginTakesAsLongForAnUnknownName holds the medians within a factor of
two; the Account form's current-password check has its own limiter, per
user (AllowPasswordChange); a sign-in destroys every session the request
presents before it issues its own.

Step 2 (L14): newHTTPServer - ReadTimeout 30 s, WriteTimeout 60 s,
IdleTimeout 2 min beside ReadHeaderTimeout; the full backup moves its write
deadline to two hours through http.ResponseController (e2e: the log shows
no failure to move it).

Step 3 (M11), read in htmx 2.0.4's source (dev/upstream/htmx-2.0.4, the npm
build byte-equal to the shipped htmx.min.js): the poll passes source: box,
and the next refresh is scheduled from htmx:afterRequest, which htmx fires
on load, error, abort and timeout, and after a swap on the nearest parent
left (the box found by detail.target's id); failures in a row double the
wait to 60 s. RequireAuth answers htmx with 401 + HX-Redirect: /login,
which htmx follows before it looks at the status. testdata/poll_test.js
runs panel.js in node against a stand-in DOM and clock (TestPanelJS; skipped
without node, which the test server lacks - run here, and it fails on the
old panel.js).

Also measured: the stage-3 e2e regression for H1 fails as it should on HEAD
with the old entrypoint ("check data/postfix/e2e_extra_map: permission
denied", the panel restarting).

Tested on the test server: go vet + go test of 25 packages, make e2e (all
pass, 639 s) on this tree.

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

cmd/panel/httpserver_test.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"testing"
+	"time"
+)
+
+// The panel's server bounds every phase a client could hold it in (audit
+// 2026-10-12, L14); before, only the header's read was.
+func TestHTTPServerHasItsTimeouts(t *testing.T) {
+	s := newHTTPServer(":0", nil)
+	for name, d := range map[string]time.Duration{
+		"ReadHeaderTimeout": s.ReadHeaderTimeout,
+		"ReadTimeout":       s.ReadTimeout,
+		"WriteTimeout":      s.WriteTimeout,
+		"IdleTimeout":       s.IdleTimeout,
+	} {
+		if d <= 0 {
+			t.Errorf("%s is %v", name, d)
+		}
+	}
+	if s.ReadTimeout < s.ReadHeaderTimeout || s.WriteTimeout < 30*time.Second {
+		t.Errorf("read %v (header %v), write %v: too short for a form or a DNS page", s.ReadTimeout, s.ReadHeaderTimeout, s.WriteTimeout)
+	}
+}