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/src/lib_commands/ui/mod.rs

@@ -1,9 +1,8 @@
 mod mini;
-mod bandsintown;
 
 pub(crate) use mini::{
     close_mini_player, open_mini_player, pause_rendering, persist_mini_pos_throttled,
     preload_mini_player, resize_mini_player, resume_rendering, set_mini_player_always_on_top,
     show_main_window, PAUSE_RENDERING_JS, RESUME_RENDERING_JS,
 };
-pub(crate) use bandsintown::fetch_bandsintown_events;
+// Bandsintown moved to psysonic-integration; lib.rs uses the full path.