feat(smartplaylist): per-playlist refreshDelay for stable daily/weekly playlists (#5790)

* feat(utils): add ParseDuration/FormatDuration with day and week units

* feat(criteria): add per-playlist refreshDelay to smart playlist rules

* feat(smartplaylist): honor per-playlist refreshDelay in refresh gate

* feat(subsonic): compute smart playlist validUntil from effective refresh delay

* fix(playlists): reset smart playlist evaluation window when rules change via API

* refactor(utils): flatten FormatDuration recursion, single-pass duration regex

* fix(playlists): address PR review feedback

- Reset EvaluatedAt to nil instead of zero-time on rules change and NSP
  re-import, so getPlaylist(s) never reports year-1 Changed/validUntil in
  the window between an edit and the next owner read
- Parse negative d/w durations so they are rejected with the consistent
  "negative duration" error instead of "unknown unit"
- Quote input in the negative-duration error, matching the parse error
- Gate per-playlist RefreshDelay behind IsSmartPlaylist, matching its doc

server/subsonic/playlists.go

@@ -168,7 +168,7 @@ func buildOSPlaylist(ctx context.Context, p model.Playlist) *responses.OpenSubso
 		pls.Readonly = true
 
 		if p.EvaluatedAt != nil {
-			pls.ValidUntil = new(p.EvaluatedAt.Add(conf.Server.SmartPlaylistRefreshDelay))
+			pls.ValidUntil = new(p.EvaluatedAt.Add(p.RefreshDelay()))
 		}
 	} else {
 		user, ok := request.UserFrom(ctx)

server/subsonic/playlists_test.go

@@ -248,6 +248,20 @@ var _ = Describe("buildPlaylist", func() {
 				Expect(result.OpenSubsonicPlaylist).To(BeNil())
 			})
 		})
+
+		Context("with a per-playlist refreshDelay", func() {
+			BeforeEach(func() {
+				playlist.Rules.RefreshDelay = 24 * time.Hour
+				player := model.Player{Client: "regular-client"}
+				ctx = request.WithPlayer(ctx, player)
+			})
+
+			It("computes validUntil from the playlist's own delay", func() {
+				result := router.buildPlaylist(ctx, playlist)
+				expected := evaluatedAt.Add(24 * time.Hour)
+				Expect(result.ValidUntil).To(Equal(&expected))
+			})
+		})
 	})
 
 	Describe("annotation leakage", func() {