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/crates/psysonic-integration/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "psysonic-integration"
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+publish = false
+
+[dependencies]
+psysonic-core = { path = "../psysonic-core" }
+
+tauri = { version = "2" }
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+tokio = { version = "1", features = ["rt", "time", "sync"] }
+reqwest = { version = "0.13", default-features = false, features = ["stream", "json", "rustls", "blocking"] }
+futures-util = "0.3"
+discord-rich-presence = "1.1"
+url = "2"
+md5 = "0.8"