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/handlers/handlers_backup.go

@@ -107,6 +107,15 @@ func (h *Handlers) HandleBackup(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("Content-Disposition", fmt.Sprintf("attachment; filename=%q", filename))
 	w.Header().Set("Cache-Control", "no-store")
 
+	// The server's write deadline is for pages (cmd/panel newHTTPServer); an
+	// archive streams for as long as it takes. Two hours carry a gigabyte at
+	// a little over 1 Mbit/s, and still end a client that keeps the
+	// connection and stops reading, which TCP alone would not (net/http, Go
+	// 1.26: ResponseController.SetWriteDeadline).
+	if err := http.NewResponseController(w).SetWriteDeadline(time.Now().Add(2 * time.Hour)); err != nil {
+		logf("panel: full backup: lift the write deadline: %v", err)
+	}
+
 	// Everything below streams: past this point headers (and possibly some
 	// bytes) are already on the wire, so a failure cannot switch to a clean
 	// error page. Log it and let the truncated download fail loudly on the