fix(portal): pre-merge security hardening from code review

- PORTAL_OPEN_LINKS now defaults OFF — /portal/open/* is an unauthenticated,
  proxy-reachable session-minting path (and a linked project's open link grants
  the whole client's scope), so it must be explicitly enabled in dev.
- Session cookie: enforce server-side expiry (check iat vs COOKIE_MAX_AGE — was
  browser-only) and guard a non-dict signed body (was an uncaught AttributeError →
  500, reachable if SECRET_KEY is the insecure default).
- Escape operator-set strings (location/rule/event names) before innerHTML +
  Leaflet tooltips — they're client-facing, so a name with markup was stored XSS
  in the client's browser. Global esc() helper applied at every injection point.
- WS _scrub_frame drops a non-JSON frame instead of forwarding it raw; /history
  rows now whitelisted like the other scoped endpoints.
- Preview-client slug uses the full project id (an 8-char prefix could collide
  two projects onto one client).

Verified: cookie reader (fresh/expired/non-dict/missing-iat) + open-links default
off; templates parse; scoped scrubbing intact.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/portal/overview.html

@@ -96,9 +96,9 @@ function updateMarker(loc) {
     const m = markersById[loc.id]; if (!m) return;
     const st = liveState[loc.id];
     m.setStyle({ fillColor: levelColor(st) });
-    let label = `<b>${loc.name}</b>`;
+    let label = `<b>${esc(loc.name)}</b>`;
     if (st) {
-        if (st.status === 'measuring') label += ` &middot; ${st.leqStr} dB Leq`;
+        if (st.status === 'measuring') label += ` &middot; ${esc(st.leqStr)} dB Leq`;
         else if (st.status === 'stopped') label += ' &middot; stopped';
         else if (st.status === 'nodevice') label += ' &middot; no device';
         else label += ' &middot; offline';
@@ -180,7 +180,7 @@ if (withCoords.length) {
             markersById[l.id] = L.circleMarker([la, lo], {
                 radius: 7, fillColor: levelColor(liveState[l.id]), color: '#fff',
                 weight: 2, opacity: 0.9, fillOpacity: 0.95,
-            }).addTo(map).bindTooltip(l.name, { direction: 'top', offset: [0, -6] });
+            }).addTo(map).bindTooltip(esc(l.name), { direction: 'top', offset: [0, -6] });
             pts.push([la, lo]);
         }
     });