chore: expose status port in Docker/compose and document http_addr

EXPOSE 8080 in the image, publish 8080 in compose, document http_addr in the
example config, and add the CPU/memory limits so a long encode can't starve
the host.

config.example.yaml

@@ -1,5 +1,9 @@
 omdb_api_key: "YOUR_API_KEY_HERE"
 
+# Status server listen address. Omit for the default ":8080"; set to "" to disable.
+# GET /status returns live encode progress, the input queue, and recent events.
+http_addr: ":8080"
+
 encoding:
   dvd:
     crf: 30