feat(portal): M2b-2 — surface alert state + breach history (internal + portal)

Internal (SLM detail page): live alarm-state badge in the Alerts header
(● N active / ✓ all clear), a History list of fired events (onset → clear, peak
dB, ack status) with an Ack button, refreshed every 20s. Reads the existing SLMM
/alerts/events + /ack via the proxy.

Portal (client, read-only, scoped): new GET /portal/api/location/{id}/events —
ownership-gated, returns a scrubbed projection (rule_name/metric/threshold/onset/
peak/clear/status only; no internal ids or ack-by) plus an `active` count. The
location page shows a red "Currently above threshold" banner when active and a
read-only breach history, polled every 20s. No ack on the client side.

Verified: portal.py compiles; both scripts balance; both templates parse.

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

backend/routers/portal.py

@@ -214,6 +214,34 @@ async def portal_location_history(location_id: str, hours: float = 2.0,
     return {"status": "ok", "readings": (r.json() or {}).get("readings", [])}
 
 
+# Whitelist of alert-event fields exposed to a client (no internal ids/ack-by).
+_PORTAL_EVENT_FIELDS = ("rule_name", "metric", "threshold_db", "onset_at",
+                        "onset_value", "peak_value", "clear_at", "status")
+
+
+@router.get("/api/location/{location_id}/events")
+async def portal_location_events(location_id: str, limit: int = 20,
+                                 client: Client = Depends(get_current_client),
+                                 db: Session = Depends(get_db)):
+    """Scrubbed breach history for a location the client owns (read-only)."""
+    resolve_client_location(client, location_id, db)
+    unit_id = active_unit_for_location(location_id, db)
+    if not unit_id:
+        return {"status": "ok", "events": []}
+    limit = max(1, min(limit, 100))
+    try:
+        async with httpx.AsyncClient(timeout=5.0) as hc:
+            r = await hc.get(f"{SLMM_BASE_URL}/api/nl43/{unit_id}/alerts/events",
+                             params={"limit": limit})
+    except Exception:
+        return {"status": "ok", "events": []}
+    if r.status_code != 200:
+        return {"status": "ok", "events": []}
+    raw = (r.json() or {}).get("events", [])
+    events = [{k: e.get(k) for k in _PORTAL_EVENT_FIELDS} for e in raw]
+    return {"status": "ok", "events": events, "active": sum(1 for e in events if e.get("status") == "active")}
+
+
 # -- live stream (fan-out feed, scoped + scrubbed) ---------------------------
 
 def _scrub_frame(raw: str) -> str: