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>

db/migrations/20260117201522_add_avg_rating_column.sql

@@ -0,0 +1,23 @@
+-- +goose Up
+ALTER TABLE album ADD COLUMN average_rating REAL NOT NULL DEFAULT 0;
+ALTER TABLE media_file ADD COLUMN average_rating REAL NOT NULL DEFAULT 0;
+ALTER TABLE artist ADD COLUMN average_rating REAL NOT NULL DEFAULT 0;
+
+-- Populate average_rating from existing ratings
+UPDATE album SET average_rating = coalesce(
+    (SELECT round(avg(rating), 2) FROM annotation WHERE item_id = album.id AND item_type = 'album' AND rating > 0),
+    0
+);
+UPDATE media_file SET average_rating = coalesce(
+    (SELECT round(avg(rating), 2) FROM annotation WHERE item_id = media_file.id AND item_type = 'media_file' AND rating > 0),
+    0
+);
+UPDATE artist SET average_rating = coalesce(
+    (SELECT round(avg(rating), 2) FROM annotation WHERE item_id = artist.id AND item_type = 'artist' AND rating > 0),
+    0
+);
+
+-- +goose Down
+ALTER TABLE artist DROP COLUMN average_rating;
+ALTER TABLE media_file DROP COLUMN average_rating;
+ALTER TABLE album DROP COLUMN average_rating;