fix(cover): embed fanart.tv project key so from-source builds work (#1139)

The project key was injected only at our build time (CI secret / option_env!),
so AUR, Nix and any from-source build had no key baked in and the External
Artwork toggle was inert there. Commit it as a source literal (like the Last.fm
key, and as fanart.tv terms expect: the app ships a project key, users add
their own on top via BYOK). Drops the now-redundant CI wiring.

src-tauri/src/cover_cache/mod.rs

@@ -135,9 +135,9 @@ pub struct CoverCacheEnsureArgs {
     /// with the album/artist name. On-demand UI ensures leave it `None`.
     #[serde(default)]
     pub library_server_id: Option<String>,
-    /// Image-scraper spike (§16 P0): when true, an artist `fanart` ensure may
-    /// fetch a fanart.tv background into `{tier}-fanart.webp`. Inert unless the
-    /// fanart project key is present (`PSYSONIC_FANART_KEY`). Off by default.
+    /// External artwork (§16): when true, an artist `fanart`/`banner` ensure may
+    /// fetch from fanart.tv into `{tier}-{provider}.webp`. Gated by the master
+    /// toggle (off by default); the project key is embedded (`FANART_PROJECT_KEY`).
     #[serde(default)]
     pub external_artwork_enabled: bool,
     /// Surface intent for external artwork — `fanart` for the 16:9 artist