feat(db): add M2 likes schema (general_likes, _albums, _artists)

Three tables keyed on (user_id, entity_id) with liked_at. Per-table
indexes on (user_id, liked_at DESC) for the recently-liked feed.
sqlc queries cover like/unlike/list-rows/count/list-ids per entity.

internal/db/migrations/0006_likes.up.sql

@@ -0,0 +1,27 @@
+-- M2 likes: three tables, one per entity type. Schema follows the spec §5
+-- pattern (general_likes was originally track-only); this slice extends it
+-- to albums and artists for full Subsonic star/unstar support.
+
+CREATE TABLE general_likes (
+    user_id   uuid        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    track_id  uuid        NOT NULL REFERENCES tracks(id) ON DELETE CASCADE,
+    liked_at  timestamptz NOT NULL DEFAULT now(),
+    PRIMARY KEY (user_id, track_id)
+);
+CREATE INDEX general_likes_user_liked_at_idx ON general_likes (user_id, liked_at DESC);
+
+CREATE TABLE general_likes_albums (
+    user_id   uuid        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    album_id  uuid        NOT NULL REFERENCES albums(id) ON DELETE CASCADE,
+    liked_at  timestamptz NOT NULL DEFAULT now(),
+    PRIMARY KEY (user_id, album_id)
+);
+CREATE INDEX general_likes_albums_user_liked_at_idx ON general_likes_albums (user_id, liked_at DESC);
+
+CREATE TABLE general_likes_artists (
+    user_id    uuid        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    artist_id  uuid        NOT NULL REFERENCES artists(id) ON DELETE CASCADE,
+    liked_at   timestamptz NOT NULL DEFAULT now(),
+    PRIMARY KEY (user_id, artist_id)
+);
+CREATE INDEX general_likes_artists_user_liked_at_idx ON general_likes_artists (user_id, liked_at DESC);