feat(library): hide games from lists and stats

src/main/resources/db/migration/V12__hidden_games.sql

@@ -0,0 +1,5 @@
+-- A game the user hid (e.g. a playtest or a tool). It stays owned and synced, but leaves every list and number.
+ALTER TABLE user_game ADD COLUMN hidden_at TIMESTAMPTZ;
+
+DROP INDEX idx_user_game_user_active;
+CREATE INDEX idx_user_game_user_active ON user_game (user_id) WHERE removed_at IS NULL AND hidden_at IS NULL;