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>

internal/web/middleware.go

@@ -12,6 +12,13 @@ const usernameKey ctxKey = 0
 // requireAuth wraps a handler so only requests with a valid session cookie
 // reach it; everyone else is redirected to the login page. The authenticated
 // username is stashed in the request context for downstream handlers.
+//
+// It also extends the sliding session (plan B.1) on activity, defined as
+// everything except a GET request carrying HX-Request: the four monitoring
+// fragments (/status/fragment, /queue/body, /logtail/body, /sendlog/rows)
+// poll every 5s regardless of whether anyone is looking at the tab, so
+// counting those as activity would make "N days idle" mean "N days since a
+// browser tab was last open" instead.
 func (s *Server) requireAuth(next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		token, ok := s.sessionToken(r)
@@ -24,11 +31,20 @@ func (s *Server) requireAuth(next http.Handler) http.Handler {
 			http.Redirect(w, r, "/login", http.StatusSeeOther)
 			return
 		}
+		if isSessionActivity(r) && s.sessions.Touch(token) {
+			s.setSessionCookie(w, token)
+		}
 		ctx := context.WithValue(r.Context(), usernameKey, username)
 		next.ServeHTTP(w, r.WithContext(ctx))
 	})
 }
 
+// isSessionActivity reports whether a request counts as administrator
+// activity for the sliding session timeout, per requireAuth's doc comment.
+func isSessionActivity(r *http.Request) bool {
+	return !(r.Method == http.MethodGet && r.Header.Get("HX-Request") != "")
+}
+
 // currentUser returns the authenticated username from the request context.
 func currentUser(r *http.Request) string {
 	if v, ok := r.Context().Value(usernameKey).(string); ok {