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>

internal/web/auth/middleware.go

@@ -11,17 +11,17 @@ func (m *Module) RequireAuth(next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		token, ok := m.sessionToken(r)
 		if !ok {
-			http.Redirect(w, r, "/login", http.StatusSeeOther)
+			toLogin(w, r)
 			return
 		}
 		userID, ok := m.sessions.Lookup(token)
 		if !ok {
-			http.Redirect(w, r, "/login", http.StatusSeeOther)
+			toLogin(w, r)
 			return
 		}
 		u, err := m.store.GetUser(userID)
 		if err != nil {
-			http.Redirect(w, r, "/login", http.StatusSeeOther)
+			toLogin(w, r)
 			return
 		}
 		// Only a session whose user exists is kept alive by being used.
@@ -34,6 +34,21 @@ func (m *Module) RequireAuth(next http.Handler) http.Handler {
 	})
 }
 
+// toLogin sends a request without a session to the sign-in page. A browser
+// gets a redirect; htmx gets HX-Redirect, which it follows with the whole
+// page, before it looks at the status (htmx 2.0.4, src/htmx.js
+// handleAjaxResponse(): location.href = the header's value). A 303 to htmx
+// was followed inside the request, and the sign-in page swapped into the box
+// that asked (audit 2026-10-12, M11).
+func toLogin(w http.ResponseWriter, r *http.Request) {
+	if r.Header.Get("HX-Request") != "" {
+		w.Header().Set("HX-Redirect", "/login")
+		w.WriteHeader(http.StatusUnauthorized)
+		return
+	}
+	http.Redirect(w, r, "/login", http.StatusSeeOther)
+}
+
 // isSessionActivity: a GET from htmx is a background poll - every panel hx-get
 // is a load or a poll - and does not renew the session. htmx 2.0.4, src/htmx.js
 // getHeaders(): every request it issues carries HX-Request: true (and