feat(subsonic): Add avgRating from subsonic spec (#4900)

* feat(subsonic): add averageRating to API responses

Add averageRating attribute to Subsonic API responses for artists,
albums, and songs. The average is calculated across all user ratings.

* perf(db): add index for average rating queries

Add composite index on (item_id, item_type, rating) to optimize
the correlated subquery used for calculating average ratings.

Signed-off-by: Terry Raimondo <terry.raimondo@gmail.com>

* test: add tests for averageRating feature

Add tests for:
- Album.AverageRating calculation in persistence layer
- MediaFile.AverageRating calculation in persistence layer
- AverageRating mapping in subsonic response helpers

Signed-off-by: Terry Raimondo <terry.raimondo@gmail.com>

* test: improve averageRating rounding test with 3 users

Add third test user to fixtures and update rounding test to use
3 ratings (5 + 4 + 4) / 3 = 4.33 for proper decimal rounding coverage.

Signed-off-by: Terry Raimondo <terry.raimondo@gmail.com>

* perf: store avg_rating on entity tables instead of using subquery

- Add avg_rating column to album, media_file, and artist tables
- Update SetRating() to recalculate and store average when ratings change
- Read avg_rating directly from entity table in withAnnotation()
- Remove old annotation index migration (no longer needed)

This trades write-time computation for read-time performance by
pre-computing the average rating instead of using a correlated
subquery on every read.

* feat: add Subsonic.EnableAverageRating config option (default true)

Allow administrators to disable exposing averageRating in Subsonic API
responses if they don't want to expose other users' rating data.

The avg_rating column is still updated internally when users rate items,
but the value is only included in API responses when this option is enabled.

* address PR comments

- Use structs:"avg_rating" with db:"avg_rating" tag instead of SQL alias
- Remove avg_rating indexes (not needed)
- Populate avg_rating columns from existing ratings in migration

* Woops

* rename avg_rating column to average_rating

---------

Signed-off-by: Terry Raimondo <terry.raimondo@gmail.com>

server/subsonic/helpers_test.go

@@ -456,4 +456,131 @@ var _ = Describe("helpers", func() {
 			})
 		})
 	})
+
+	Describe("AverageRating in responses", func() {
+		var ctx context.Context
+
+		BeforeEach(func() {
+			ctx = context.Background()
+			conf.Server.Subsonic.EnableAverageRating = true
+		})
+
+		Describe("childFromMediaFile", func() {
+			It("includes averageRating when set", func() {
+				mf := model.MediaFile{
+					ID:    "mf-avg-1",
+					Title: "Test Song",
+					Annotations: model.Annotations{
+						AverageRating: 4.5,
+					},
+				}
+				child := childFromMediaFile(ctx, mf)
+				Expect(child.AverageRating).To(Equal(4.5))
+			})
+
+			It("returns 0 for averageRating when not set", func() {
+				mf := model.MediaFile{
+					ID:    "mf-avg-2",
+					Title: "Test Song No Rating",
+				}
+				child := childFromMediaFile(ctx, mf)
+				Expect(child.AverageRating).To(Equal(0.0))
+			})
+		})
+
+		Describe("childFromAlbum", func() {
+			It("includes averageRating when set", func() {
+				al := model.Album{
+					ID:   "al-avg-1",
+					Name: "Test Album",
+					Annotations: model.Annotations{
+						AverageRating: 3.75,
+					},
+				}
+				child := childFromAlbum(ctx, al)
+				Expect(child.AverageRating).To(Equal(3.75))
+			})
+
+			It("returns 0 for averageRating when not set", func() {
+				al := model.Album{
+					ID:   "al-avg-2",
+					Name: "Test Album No Rating",
+				}
+				child := childFromAlbum(ctx, al)
+				Expect(child.AverageRating).To(Equal(0.0))
+			})
+		})
+
+		Describe("toArtist", func() {
+			It("includes averageRating when set", func() {
+				conf.Server.Subsonic.EnableAverageRating = true
+				r := httptest.NewRequest("GET", "/test", nil)
+				a := model.Artist{
+					ID:   "ar-avg-1",
+					Name: "Test Artist",
+					Annotations: model.Annotations{
+						AverageRating: 5.0,
+					},
+				}
+				artist := toArtist(r, a)
+				Expect(artist.AverageRating).To(Equal(5.0))
+			})
+		})
+
+		Describe("toArtistID3", func() {
+			It("includes averageRating when set", func() {
+				conf.Server.Subsonic.EnableAverageRating = true
+				r := httptest.NewRequest("GET", "/test", nil)
+				a := model.Artist{
+					ID:   "ar-avg-2",
+					Name: "Test Artist ID3",
+					Annotations: model.Annotations{
+						AverageRating: 2.5,
+					},
+				}
+				artist := toArtistID3(r, a)
+				Expect(artist.AverageRating).To(Equal(2.5))
+			})
+		})
+
+		Describe("EnableAverageRating config", func() {
+			It("excludes averageRating when disabled", func() {
+				conf.Server.Subsonic.EnableAverageRating = false
+
+				mf := model.MediaFile{
+					ID:    "mf-cfg-1",
+					Title: "Test Song",
+					Annotations: model.Annotations{
+						AverageRating: 4.5,
+					},
+				}
+				child := childFromMediaFile(ctx, mf)
+				Expect(child.AverageRating).To(Equal(0.0))
+
+				al := model.Album{
+					ID:   "al-cfg-1",
+					Name: "Test Album",
+					Annotations: model.Annotations{
+						AverageRating: 3.75,
+					},
+				}
+				albumChild := childFromAlbum(ctx, al)
+				Expect(albumChild.AverageRating).To(Equal(0.0))
+
+				r := httptest.NewRequest("GET", "/test", nil)
+				a := model.Artist{
+					ID:   "ar-cfg-1",
+					Name: "Test Artist",
+					Annotations: model.Annotations{
+						AverageRating: 5.0,
+					},
+				}
+				artist := toArtist(r, a)
+				Expect(artist.AverageRating).To(Equal(0.0))
+
+				artistID3 := toArtistID3(r, a)
+				Expect(artistID3.AverageRating).To(Equal(0.0))
+			})
+		})
+	})
 })