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.down.sql

@@ -0,0 +1,3 @@
+DROP TABLE IF EXISTS general_likes_artists;
+DROP TABLE IF EXISTS general_likes_albums;
+DROP TABLE IF EXISTS general_likes;