refactor: extract psysonic-integration crate (M5/7)

Moves all outbound external-service bridges out of the top crate into a
new psysonic-integration crate.

  crates/psysonic-integration/
    src/discord.rs                     Rich Presence + iTunes artwork
    src/navidrome/{client,covers,...}  Native REST API admin (5 modules)
    src/remote.rs                      radio-browser, last.fm, ICY meta,
                                       generic CORS proxy (fetch_url_bytes
                                       / fetch_json_url / resolve_stream_url)
    src/bandsintown.rs                 events for an artist
    src/lib.rs                         module declarations + macro re-exports

Top crate keeps `crate::discord` working via
`pub use psysonic_integration::discord;`.

invoke_handler! in lib.rs uses full deepest paths
(`psysonic_integration::navidrome::users::nd_list_users`, etc.) so
Tauri's `__cmd__*` magic macros resolve at the right module — same
pattern audio + syncfs already use.

Cross-crate refs migrated:
  crate::subsonic_wire_user_agent → psysonic_core::user_agent::*
  pub(crate) → pub                  (cross-crate visibility)

Behaviour preserving. Cargo check + clippy --workspace clean.

src-tauri/Cargo.lock

@@ -3611,6 +3611,7 @@ dependencies = [
  "psysonic-analysis",
  "psysonic-audio",
  "psysonic-core",
+ "psysonic-integration",
  "psysonic-syncfs",
  "reqwest",
  "ringbuf",
@@ -3695,6 +3696,22 @@ dependencies = [
  "tauri",
 ]
 
+[[package]]
+name = "psysonic-integration"
+version = "1.46.0-dev"
+dependencies = [
+ "discord-rich-presence",
+ "futures-util",
+ "md5",
+ "psysonic-core",
+ "reqwest",
+ "serde",
+ "serde_json",
+ "tauri",
+ "tokio",
+ "url",
+]
+
 [[package]]
 name = "psysonic-syncfs"
 version = "1.46.0-dev"