feat: persist and expose plugin playlist ValidUntil in Subsonic API

Add a ValidUntil field to the Playlist model and persist it from the
plugin's GetPlaylistResponse during sync. This allows clients to know
when a plugin playlist's data will be refreshed. The value is exposed
in the OpenSubsonic playlist response alongside the existing
smart playlist ValidUntil calculation. The migration is consolidated
into a single multi-statement ExecContext call.

plugins/playlist_provider.go

@@ -200,6 +200,10 @@ func (p *playlistSyncer) syncPlaylist(info capabilities.PlaylistInfo, dbID strin
 		PluginID:         p.pluginName,
 		PluginPlaylistID: info.ID,
 	}
+	if resp.ValidUntil > 0 {
+		t := time.Unix(resp.ValidUntil, 0)
+		pls.ValidUntil = &t
+	}
 
 	// Set tracks from matched media files
 	pls.AddMediaFiles(matched)

plugins/playlist_provider_test.go

@@ -75,6 +75,7 @@ var _ = Describe("PlaylistProvider", Ordered, func() {
 			Expect(dailyMix1.PluginID).To(Equal("test-playlist-provider"))
 			Expect(dailyMix1.PluginPlaylistID).To(Equal("daily-mix-1"))
 			Expect(dailyMix1.Public).To(BeFalse())
+			Expect(dailyMix1.ValidUntil).To(BeNil())
 		})
 
 		It("generates deterministic playlist IDs", func() {