ADS-B map: altitude-colored aircraft icons + click-to-show flight trail

Icons were 16px accent-colored glyphs, indistinguishable from OSM's own
airport symbols. Now a 30px outlined airliner silhouette filled on
tar1090/ADS-B Exchange's altitude hue ramp, with a callsign/altitude
hover tooltip; the selected aircraft grows and gets a white outline.

Clicking an aircraft draws the path heard so far, segment-colored by
altitude. c2-core writes one point per position change to
aircraft/{icao}/positions (deduped in-process, writes now concurrent);
points carry expire_at and a TTL fieldOverride deletes them after ~24h.
Trail reads are gated on the parent aircraft doc's org via get(), so the
query needs no org filter or composite index. The latest stretch without
a 20-min gap counts as the current flight.

Verified: c2-core pytest 479 passed; frontend tsc --noEmit clean (node:20
container on radio-box).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

infra/firestore/firestore.indexes.json

@@ -77,5 +77,13 @@
       ]
     }
   ],
-  "fieldOverrides": []
+  "fieldOverrides": [
+    {
+      "//": "TTL: flight-trail points (aircraft/{icao}/positions, server-26 telemetry.py) are deleted ~24h after expire_at. indexes: [] because nothing queries on expire_at.",
+      "collectionGroup": "positions",
+      "fieldPath": "expire_at",
+      "ttl": true,
+      "indexes": []
+    }
+  ]
 }