feat: implement B.1 — persist login sessions in SQLite with sliding idle timeout

Sessions move from an in-memory map (absolute 12h TTL) to a `sessions`
table (migration 0002), storing only the SHA-256 of the token. Expiry is
now a sliding idle window (PANEL_SESSION_IDLE_DAYS, default 7, no
absolute cap), extended at most once an hour and never by the
monitoring screens' background polling (GET + HX-Request), so a
forgotten open tab doesn't keep a session alive indefinitely. A login
now survives a container restart or redeploy.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

cmd/panel/httpserver.go

@@ -37,6 +37,7 @@ func serveHTTP(ctx context.Context, cfg config, st *store.Store) error {
 		TLSCertFile:       cfg.tlsCertFile,
 		OpenDKIMSocket:    cfg.opendkimSocket,
 		JournalSocket:     cfg.journalSocket,
+		SessionIdleDays:   cfg.sessionIdleDays,
 	}, cfg.setupTokenPath)
 	if err != nil {
 		return err