✨ Add cover proxy for external LAN media servers

- Implement new /covers/proxy endpoint to cache and rewrite external LAN cover URLs
- Add url/urlencoding dependencies for URL parsing/escaping in pmocontrol and pmocovers Cargo.toml
- Transform album_art_uri fields to use proxy endpoint for LAN URLs in REST and SSE handlers (browse_container, MetadataChanged)
- Add URL validation logic to detect LAN vs public URLs and avoid self-caching
- Update Cargo.lock with new dependencies

Cargo.lock

@@ -4016,6 +4016,8 @@ dependencies = [
  "tracing-log 0.1.4",
  "tracing-subscriber",
  "ureq",
+ "url",
+ "urlencoding",
  "utoipa",
  "xmltree 0.11.0",
 ]
@@ -4038,6 +4040,7 @@ dependencies = [
  "tempfile",
  "tokio",
  "tracing",
+ "url",
  "utoipa",
  "webp",
 ]
@@ -6550,6 +6553,12 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "urlencoding"
+version = "2.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "daf8dba3b7eb870caf1ddeed7bc9d2a049f3cfdfae7cb521b087cc33ae4c49da"
+
 [[package]]
 name = "users"
 version = "0.11.0"