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>

persistence/mediafile_repository_test.go

@@ -157,6 +157,74 @@ var _ = Describe("MediaRepository", func() {
 			Expect(mf.PlayCount).To(Equal(int64(1)))
 		})
 
+		Describe("AverageRating", func() {
+			var raw *mediaFileRepository
+
+			BeforeEach(func() {
+				raw = mr.(*mediaFileRepository)
+			})
+
+			It("returns 0 when no ratings exist", func() {
+				newID := id.NewRandom()
+				Expect(mr.Put(&model.MediaFile{LibraryID: 1, ID: newID, Path: "/test/no-rating.mp3"})).To(Succeed())
+
+				mf, err := mr.Get(newID)
+				Expect(err).ToNot(HaveOccurred())
+				Expect(mf.AverageRating).To(Equal(0.0))
+
+				_, _ = raw.executeSQL(squirrel.Delete("media_file").Where(squirrel.Eq{"id": newID}))
+			})
+
+			It("returns the user's rating as average when only one user rated", func() {
+				newID := id.NewRandom()
+				Expect(mr.Put(&model.MediaFile{LibraryID: 1, ID: newID, Path: "/test/single-rating.mp3"})).To(Succeed())
+				Expect(mr.SetRating(5, newID)).To(Succeed())
+
+				mf, err := mr.Get(newID)
+				Expect(err).ToNot(HaveOccurred())
+				Expect(mf.AverageRating).To(Equal(5.0))
+
+				_, _ = raw.executeSQL(squirrel.Delete("annotation").Where(squirrel.Eq{"item_id": newID}))
+				_, _ = raw.executeSQL(squirrel.Delete("media_file").Where(squirrel.Eq{"id": newID}))
+			})
+
+			It("calculates average across multiple users", func() {
+				newID := id.NewRandom()
+				Expect(mr.Put(&model.MediaFile{LibraryID: 1, ID: newID, Path: "/test/multi-rating.mp3"})).To(Succeed())
+
+				Expect(mr.SetRating(3, newID)).To(Succeed())
+
+				user2Ctx := request.WithUser(GinkgoT().Context(), regularUser)
+				user2Repo := NewMediaFileRepository(user2Ctx, GetDBXBuilder())
+				Expect(user2Repo.SetRating(5, newID)).To(Succeed())
+
+				mf, err := mr.Get(newID)
+				Expect(err).ToNot(HaveOccurred())
+				Expect(mf.AverageRating).To(Equal(4.0))
+
+				_, _ = raw.executeSQL(squirrel.Delete("annotation").Where(squirrel.Eq{"item_id": newID}))
+				_, _ = raw.executeSQL(squirrel.Delete("media_file").Where(squirrel.Eq{"id": newID}))
+			})
+
+			It("excludes zero ratings from average calculation", func() {
+				newID := id.NewRandom()
+				Expect(mr.Put(&model.MediaFile{LibraryID: 1, ID: newID, Path: "/test/zero-excluded.mp3"})).To(Succeed())
+
+				Expect(mr.SetRating(4, newID)).To(Succeed())
+
+				user2Ctx := request.WithUser(GinkgoT().Context(), regularUser)
+				user2Repo := NewMediaFileRepository(user2Ctx, GetDBXBuilder())
+				Expect(user2Repo.SetRating(0, newID)).To(Succeed())
+
+				mf, err := mr.Get(newID)
+				Expect(err).ToNot(HaveOccurred())
+				Expect(mf.AverageRating).To(Equal(4.0))
+
+				_, _ = raw.executeSQL(squirrel.Delete("annotation").Where(squirrel.Eq{"item_id": newID}))
+				_, _ = raw.executeSQL(squirrel.Delete("media_file").Where(squirrel.Eq{"id": newID}))
+			})
+		})
+
 		It("preserves play date if and only if provided date is older", func() {
 			id := "incplay.playdate"
 			Expect(mr.Put(&model.MediaFile{LibraryID: 1, ID: id})).To(BeNil())