feat(library): playtime history per day and a nightly sync

Each sync records the growth of playtime per game and day (Europe/Berlin).
The first successful sync of a Steam account is only the baseline, tracked in
user_steam_sync.history_steam_id, so a new or failed account never turns its
whole playtime into one day. A nightly sync at 23:55 runs the recently used
accounts one after the other. GET /api/library/activity returns minutes per
day and the games of Steam's last two weeks.

src/main/resources/db/migration/V4__playtime_history.sql

@@ -0,0 +1,21 @@
+-- Minutes played per user, game and day (Europe/Berlin), as the growth of playtime_forever between two syncs.
+-- The first sync of a Steam account is only the baseline and writes no rows.
+CREATE TABLE user_game_playtime_day (
+    user_id BIGINT  NOT NULL,
+    day     DATE    NOT NULL,
+    appid   BIGINT  NOT NULL,
+    minutes INTEGER NOT NULL CHECK (minutes > 0),
+    PRIMARY KEY (user_id, day, appid)
+);
+
+-- The Steam account whose playtime is the baseline for the history: set after a successful sync only.
+-- Growth is recorded only while it equals steam_id, so a failed first sync of a new account cannot turn
+-- that account's whole playtime into one day.
+ALTER TABLE user_steam_sync ADD COLUMN history_steam_id VARCHAR(32);
+
+-- Last request of the user. The nightly sync skips accounts nobody used for a while, e.g. after the user
+-- removed the Steam ID (Core does not learn about that).
+ALTER TABLE user_steam_sync ADD COLUMN user_seen_at TIMESTAMPTZ;
+
+-- Steam's own "last two weeks" playtime, updated on every sync.
+ALTER TABLE user_game ADD COLUMN playtime_2weeks INTEGER NOT NULL DEFAULT 0;