✨ Add pmocovers integration for external cover URL proxying

- Introduce optional `pmocover` dependency in pmocontrol
- Add cover URL transformation logic for both REST and SSE endpoints using `pmocovers::proxy_cover_url`/sync
- Refactor `/covers/proxy?...=` handler to accept any external URL and return local cached route
- Implement helper functions `transform_cover_url` (async) & sync variant for consistent cover URL normalization
- Update Cargo.lock to include `pmocovers`

Cargo.lock

@@ -3996,6 +3996,7 @@ dependencies = [
  "futures-util",
  "mdns",
  "percent-encoding",
+ "pmocovers",
  "pmodidl",
  "pmoserver",
  "pmoupnp",