feat(library): let auditors merge duplicate games

An auditor merges a game added by hand into its Steam game or another
game added by hand (V21). Every user's row moves; a user with both rows
keeps one, with the status, rating and notes set by hand. game_alias
sends later adds to the kept key, and admin_audit logs who merged what.
The game search shows games already in ToBePlayed first.

src/main/resources/db/migration/V21__game_merge.sql

@@ -0,0 +1,24 @@
+-- An auditor's merge of two library keys of the same game: from_appid (always minus an IGDB id) now lives under
+-- to_appid, so a later add of the from game lands there. Kept flat: no to_appid is also a from_appid.
+CREATE TABLE game_alias (
+    from_appid BIGINT PRIMARY KEY,
+    to_appid   BIGINT NOT NULL
+);
+
+-- One row per auditor or admin action in Core, like Auth's admin_audit. No foreign keys: the rows outlive the game
+-- rows and the users. detail keeps the names, since the from game's catalog row is deleted.
+CREATE TABLE admin_audit (
+    id            BIGSERIAL   PRIMARY KEY,
+    actor_user_id BIGINT      NOT NULL,
+    action        VARCHAR(32) NOT NULL,
+    appid         BIGINT,
+    target_appid  BIGINT,
+    detail        TEXT,
+    created_at    TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+-- A merge moves user_game rows to another appid; their status history moves with them.
+ALTER TABLE user_game_status_change
+    DROP CONSTRAINT user_game_status_change_user_id_appid_fkey,
+    ADD CONSTRAINT user_game_status_change_user_id_appid_fkey FOREIGN KEY (user_id, appid)
+        REFERENCES user_game (user_id, appid) ON DELETE CASCADE ON UPDATE CASCADE;