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/sql_annotations.go

@@ -17,7 +17,7 @@ const annotationTable = "annotation"
 func (r sqlRepository) withAnnotation(query SelectBuilder, idField string) SelectBuilder {
 	userID := loggedUser(r.ctx).ID
 	if userID == invalidUserId {
-		return query
+		return query.Columns(fmt.Sprintf("%s.average_rating", r.tableName))
 	}
 	query = query.
 		LeftJoin("annotation on ("+
@@ -38,6 +38,8 @@ func (r sqlRepository) withAnnotation(query SelectBuilder, idField string) Selec
 		query = query.Columns("coalesce(play_count, 0) as play_count")
 	}
 
+	query = query.Columns(fmt.Sprintf("%s.average_rating", r.tableName))
+
 	return query
 }
 
@@ -79,7 +81,22 @@ func (r sqlRepository) SetStar(starred bool, ids ...string) error {
 
 func (r sqlRepository) SetRating(rating int, itemID string) error {
 	ratedAt := time.Now()
-	return r.annUpsert(map[string]interface{}{"rating": rating, "rated_at": ratedAt}, itemID)
+	err := r.annUpsert(map[string]interface{}{"rating": rating, "rated_at": ratedAt}, itemID)
+	if err != nil {
+		return err
+	}
+	return r.updateAvgRating(itemID)
+}
+
+func (r sqlRepository) updateAvgRating(itemID string) error {
+	upd := Update(r.tableName).
+		Where(Eq{"id": itemID}).
+		Set("average_rating", Expr(
+			"coalesce((select round(avg(rating), 2) from annotation where item_id = ? and item_type = ? and rating > 0), 0)",
+			itemID, r.tableName,
+		))
+	_, err := r.executeSQL(upd)
+	return err
 }
 
 func (r sqlRepository) IncPlayCount(itemID string, ts time.Time) error {