firestore: add the alert_events composite index (#51)

collectionGroup alert_events (acknowledged, org_id, triggered_at desc) — the index the /watch Triggered Alerts tab and the site-wide useUnacknowledgedAlerts hook require. Still needs a manual deploy; no automation exists (#51).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Claude-Session: https://claude.ai/code/session_01Tbknwttzou4s46PAykmtix

infra/firestore/firestore.indexes.json

@@ -75,6 +75,16 @@
         { "fieldPath": "acknowledged", "order": "ASCENDING" },
         { "fieldPath": "triggered_at", "order": "ASCENDING" }
       ]
+    },
+    {
+      "//": "drb-frontend lib/useAlerts.ts useUnacknowledgedAlerts (nav badge) and the /watch \"Triggered Alerts\" tab — where(org_id ==) where(acknowledged == false) orderBy(triggered_at desc). Threw \"the query requires an index\" on every page until this was declared (server-26#51). Field tuple and triggered_at DESCENDING copy the console create_composite link in that issue verbatim, so a gcloud/console create and a firebase deploy converge on one index rather than the ASC-vs-DESC pair that caused server-26#33. Distinct from the (org_id, triggered_at) alert-feed index above (no acknowledged filter) and supersedes the pre-tenancy live alert_events(acknowledged, triggered_at) index #33 says to delete.",
+      "collectionGroup": "alert_events",
+      "queryScope": "COLLECTION",
+      "fields": [
+        { "fieldPath": "acknowledged", "order": "ASCENDING" },
+        { "fieldPath": "org_id", "order": "ASCENDING" },
+        { "fieldPath": "triggered_at", "order": "DESCENDING" }
+      ]
     }
   ],
   "fieldOverrides": []