remove built-in Spotify integration (#5197)

* refactor: remove built-in Spotify integration

Remove the Spotify adapter and all related configuration, replacing
the built-in integration with the plugin system. This deletes the
adapters/spotify package, removes Spotify config options (ID/Secret),
updates the default agents list from "deezer,lastfm,spotify" to
"deezer,lastfm", and cleans up all references across configuration,
metrics, logging, artwork caching, and documentation. Users with
Spotify config options will now see a warning that the options are
no longer available.

* feat: add ListenBrainz to list of default agents

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

plugins/manifest_test.go

@@ -19,7 +19,7 @@ var _ = Describe("Manifest", func() {
 				"permissions": {
 					"http": {
 						"reason": "Fetch metadata",
-						"requiredHosts": ["api.example.com", "*.spotify.com"]
+						"requiredHosts": ["api.example.com", "*.musicbrainz.org"]
 					}
 				}
 			}`)
@@ -34,7 +34,7 @@ var _ = Describe("Manifest", func() {
 			Expect(*m.Website).To(Equal("https://example.com"))
 			Expect(m.Permissions.Http).ToNot(BeNil())
 			Expect(*m.Permissions.Http.Reason).To(Equal("Fetch metadata"))
-			Expect(m.Permissions.Http.RequiredHosts).To(ContainElements("api.example.com", "*.spotify.com"))
+			Expect(m.Permissions.Http.RequiredHosts).To(ContainElements("api.example.com", "*.musicbrainz.org"))
 		})
 
 		It("parses a minimal manifest", func() {