fix(playlists): match REST cols case-insensitively (PR #5542 review)

Go's encoding/json populates struct fields from case-variant keys like
{"Name":"x"} or {"OwnerId":"y"}, but rest.Put's getFieldNames extracts
raw JSON keys verbatim. With case-sensitive matching, sentFields would
ignore the field on the update side — a request with {"Name":"Renamed"}
would parse into entity.Name but then sent("name") returns false and
the rename silently no-ops.

Normalize both sides to lowercase. The entity-based owner-permission
guard added in the previous commit remains as belt-and-suspenders but
is now redundant with this change.

Also clarify the applyContentUpdate doc comment: namePtr/commentPtr
are nil when the field is absent OR present-but-unchanged, while
publicPtr only tracks presence (an idempotent public is still forwarded).

core/playlists/rest_adapter.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"reflect"
+	"strings"
 
 	"github.com/deluan/rest"
 	"github.com/navidrome/navidrome/model"
@@ -126,8 +127,10 @@ func (s *playlists) updatePlaylistEntity(ctx context.Context, id string, entity
 
 // applyContentUpdate handles updates that change at least one of name/comment/
 // owner/rules. It goes through updateMetadata, which always bumps updatedAt
-// (invalidating cached cover-art URLs). Pointer args are nil for fields not
-// present in the request.
+// (invalidating cached cover-art URLs). namePtr/commentPtr are nil when the
+// field is absent from the request OR present-but-unchanged (so updateMetadata
+// skips them); publicPtr is nil only when public is absent from the request
+// (an idempotent public value is still forwarded).
 func (s *playlists) applyContentUpdate(ctx context.Context, current, entity *model.Playlist,
 	sent func(string) bool, nameChanged, commentChanged, ownerChanged, rulesChanged bool,
 ) error {
@@ -175,15 +178,17 @@ func (s *playlists) applyFlagsOnly(ctx context.Context, current, entity *model.P
 }
 
 // sentFields returns a predicate that reports whether a JSON field was present
-// in the request body. An empty cols list means "treat the entity as a full
-// record" — every field is considered sent.
+// in the request body. Matching is case-insensitive to mirror Go's json
+// decoder, which populates struct fields from case-variant keys like
+// {"Name":"x"} or {"OWNERID":"y"}. An empty cols list means "treat the entity
+// as a full record" — every field is considered sent.
 func sentFields(cols []string) func(string) bool {
 	if len(cols) == 0 {
 		return func(string) bool { return true }
 	}
-	set := slice.ToMap(cols, func(c string) (string, struct{}) { return c, struct{}{} })
+	set := slice.ToMap(cols, func(c string) (string, struct{}) { return strings.ToLower(c), struct{}{} })
 	return func(field string) bool {
-		_, ok := set[field]
+		_, ok := set[strings.ToLower(field)]
 		return ok
 	}
 }

core/playlists/rest_adapter_test.go

@@ -367,6 +367,18 @@ var _ = Describe("REST Adapter", func() {
 					err := repo.Update("partial", &model.Playlist{Public: true}, "public")
 					Expect(err).ToNot(HaveOccurred())
 				})
+
+				It("matches cols case-insensitively (mirrors json decoder behavior)", func() {
+					// Go's json decoder populates struct fields from case-variant keys
+					// like {"Name":"x"}, but rest.Put's field-name extraction is
+					// case-sensitive. sentFields normalizes both sides so a request
+					// with {"Name":"Renamed"} is honored, not silently ignored.
+					repo = ps.NewRepository(ctx).(rest.Persistable)
+					err := repo.Update("partial", &model.Playlist{Name: "Renamed"}, "Name")
+					Expect(err).ToNot(HaveOccurred())
+					Expect(mockPlsRepo.Last.Name).To(Equal("Renamed"))
+					Expect(mockPlsRepo.Last.Comment).To(Equal("Original comment"))
+				})
 			})
 		})