511NY DOT cameras + traffic events map layers (#183)

GET /traffic/511 serves a bbox slice of the statewide 511NY camera and event
feeds from an in-memory cache (cameras 1h, events 2m TTL, fetched lazily) --
public data, so no Firestore writes. A failed refresh keeps the last good data
and reports the error; a schema change (nothing parses) is an error, not an
empty layer. Frontend: opt-in "DOT Cameras" and "Traffic Events" overlays that
fetch only while shown, with an on-map notice when the feed is down or stale.
NY511_API_KEY is optional in config (the API answers without one today; the
terms require a registered key).

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

drb-c2-core/.env.example

@@ -26,6 +26,7 @@ NODE_OFFLINE_THRESHOLD=90
 # Google Maps — for geocoding location strings extracted from transcripts
 # Enable "Geocoding API" in Cloud Console for this key
 GOOGLE_MAPS_API_KEY=
+NY511_API_KEY=
 
 # OpenAI — for transcription (Whisper), intelligence extraction, embeddings, and summaries
 OPENAI_API_KEY=