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-frontend/lib/use511.ts

@@ -0,0 +1,57 @@
+"use client";
+
+import { useEffect, useRef, useState } from "react";
+import type L from "leaflet";
+import { c2api } from "@/lib/c2api";
+import type { Ny511Response } from "@/lib/types";
+
+// 511NY cameras/events for the current map view (server-26#183). Unlike
+// useAircraft/useVessels this is not a Firestore listener: the data is public
+// and statewide, so c2-core caches it in memory and serves a bbox slice.
+// Fetches only while `enabled` (the overlay is on), on pan/zoom (debounced),
+// and on a poll matching the server's events TTL.
+const POLL_MS = 2 * 60 * 1000;
+const MOVE_DEBOUNCE_MS = 400;
+const BBOX_PAD = 0.2;   // fetch a little past the edges so small pans don't blank the layer
+
+export function use511(map: L.Map, layers: "cameras" | "events", enabled: boolean) {
+  const [data, setData] = useState<Ny511Response>({});
+  const [error, setError] = useState<string | null>(null);
+  const seq = useRef(0);
+
+  useEffect(() => {
+    if (!enabled) { setData({}); return; }
+    let debounce: ReturnType<typeof setTimeout> | undefined;
+
+    const load = async () => {
+      const b = map.getBounds().pad(BBOX_PAD);
+      const mine = ++seq.current;
+      try {
+        const res = await c2api.get511(
+          { south: b.getSouth(), west: b.getWest(), north: b.getNorth(), east: b.getEast() },
+          layers,
+        );
+        if (mine !== seq.current) return;   // a newer pan's response wins
+        setData(res);
+        setError(null);
+      } catch (e) {
+        if (mine !== seq.current) return;
+        console.error("use511:", e);
+        setError(e instanceof Error ? e.message : String(e));
+      }
+    };
+    const onMove = () => { clearTimeout(debounce); debounce = setTimeout(load, MOVE_DEBOUNCE_MS); };
+
+    load();
+    map.on("moveend", onMove);
+    const poll = setInterval(load, POLL_MS);
+    return () => {
+      map.off("moveend", onMove);
+      clearTimeout(debounce);
+      clearInterval(poll);
+      seq.current++;   // drop any in-flight response
+    };
+  }, [map, layers, enabled]);
+
+  return { data, error };
+}