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>

backend/routers/portal.py

@@ -211,7 +211,9 @@ async def portal_location_history(location_id: str, hours: float = 2.0,
         return {"status": "ok", "readings": []}
     if r.status_code != 200:
         return {"status": "ok", "readings": []}
-    return {"status": "ok", "readings": (r.json() or {}).get("readings", [])}
+    raw = (r.json() or {}).get("readings", [])
+    fields = ("timestamp", "lp", "leq", "lmax", "ln1", "ln2")  # whitelist, like the other endpoints
+    return {"status": "ok", "readings": [{k: x.get(k) for k in fields} for x in raw]}
 
 
 # Whitelist of alert-event fields exposed to a client (no internal ids/ack-by).
@@ -272,14 +274,15 @@ async def portal_location_thresholds(location_id: str,
 
 # -- live stream (fan-out feed, scoped + scrubbed) ---------------------------
 
-def _scrub_frame(raw: str) -> str:
+def _scrub_frame(raw: str):
     """Project a monitor frame down to the portal whitelist. Drops internal fields
     (unit_id, raw_payload, lmin) before it reaches a client; passes control fields
-    (feed_status, heartbeat) + timestamp through."""
+    (feed_status, heartbeat) + timestamp through. Returns None for a non-JSON frame
+    so the caller drops it rather than forwarding anything unscrubbed."""
     try:
         d = json.loads(raw)
     except Exception:
-        return raw
+        return None
     out = {k: d.get(k) for k in _PORTAL_LIVE_FIELDS if k in d}
     if "timestamp" in d:
         out["timestamp"] = d["timestamp"]
@@ -327,7 +330,9 @@ async def portal_location_stream(websocket: WebSocket, location_id: str):
 
         async def forward_to_client():
             async for message in backend_ws:
-                await websocket.send_text(_scrub_frame(message))
+                frame = _scrub_frame(message)
+                if frame is not None:
+                    await websocket.send_text(frame)
 
         async def watch_client():
             while True: