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/app/config.py

@@ -32,6 +32,9 @@ class Settings(BaseSettings):
 
     # Google Maps (geocoding)
     google_maps_api_key: Optional[str] = None
+    # 511NY developer key (server-26#183). Optional: the API answered without one
+    # as of 2026-09-27, but its terms require a registered key.
+    ny511_api_key: Optional[str] = None
 
     # Gemini (intelligence extraction, embeddings, incident summaries)
     gemini_api_key: Optional[str] = None