Add basic ping server to ingest

Primarily for uptime tracking.

deno.json

···
   },
   "tasks": {
     "start-feedgen": "deno serve --no-prompt --env-file --allow-env --allow-ffi --allow-read --port=4000 feedgen.ts",
-    "start-ingest": "deno run --no-prompt --env-file --allow-net --allow-env --allow-ffi --allow-read ingest.ts"
+    "start-ingest": "deno serve --no-prompt --env-file --allow-net --allow-env --allow-ffi --allow-read --port=4001 ingest.ts"
   }
 }

ingest.ts

···
 });
 
 jetstream.start();
+
+export default {
+  fetch() {
+    return new Response("Pong!");
+  }
+} satisfies Deno.ServeDefaultExport;