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

cmd/panel/main.go

@@ -60,6 +60,7 @@ type config struct {
 	cookieSecure      bool
 	submissionEnabled bool
 	trustedProxies    []*net.IPNet
+	sessionIdleDays   int
 
 	// Read-only inputs to the panel's status page: the certificate Postfix
 	// serves and the two milter sockets it connects to. The defaults mirror
@@ -103,6 +104,9 @@ func loadConfig() config {
 		// XFF header is trivially forgeable, so it's ignored unless the panel is
 		// told which proxy to trust.
 		trustedProxies: parseTrustedProxies(os.Getenv("TRUSTED_PROXY_CIDR")),
+		// Sliding session idle timeout (spec 7.6.6, plan B.1). Non-positive/invalid
+		// falls back to the 7-day default inside internal/web.
+		sessionIdleDays: envInt("PANEL_SESSION_IDLE_DAYS", 7),
 
 		tlsCertFile:    envDefault("TLS_CERT_FILE", "/etc/postfix/tls/fullchain.pem"),
 		opendkimSocket: envDefault("OPENDKIM_SOCKET", "/run/opendkim/opendkim.sock"),