feat(audit): undo button for merges, hide manual games

The audit log shows Undo on a merge row for 30 days, with a confirm
dialog and clear errors. The details page follows a merged key to the
kept game. Hide and Show now work for manual games too. de/en texts.

README.md

@@ -81,7 +81,7 @@ To Be Played is split into three repositories:
 - **Finished?**: Core marks a game as finished by itself when the ending achievement is unlocked and the status was
   never set by hand. Otherwise the game page asks: "You unlocked the ending, finished?" or "You played longer than
   most players need, finished?", with "Mark as finished" and "Not yet".
-- **Hide games**: a Steam game (e.g. a playtest or a tool) can be hidden on its detail page. It leaves the lists,
+- **Hide games**: a game, Steam or added by hand (e.g. a playtest or a tool), can be hidden on its detail page. It leaves the lists,
   the dashboard, the stats and the public profile; the library's "Hidden" filter shows it again for unhiding.
 - **Wishlist** with store price and discount.
 - **Steam account** linked in the settings with "Sign in through Steam": the user signs in on Steam and Auth gets
@@ -131,13 +131,13 @@ To Be Played is split into three repositories:
 - **Correct game data** (AUDITOR or higher): "Correct data" on the game detail page changes name, header image,
   speedrun link, moods, endless flag, genres and time to beat (hours and minutes) for every user; each field goes back to the source on its own.
   "Merge duplicate" joins two entries of the same game (e.g. the Steam game and the same game added by hand) for
-  every user.
+  every user; an auditor can undo a merge for 30 days from the audit log ("Undo"). An old link of a merged game opens the kept game.
 - **Game catalog** (`/audit/games`, AUDITOR or higher): search every game any user has by name (cover, platforms,
   key), paged; "Correct" opens the correction dialog, also for a game the auditor does not own. The audit log links
   its games here.
 - **Link queue** (`/audit/links`, AUDITOR or higher): the pairs IGDB suggests and users asked for, with "requested by N
-  players"; Link or Dismiss; paged, 20 pairs a page. **Audit log** (`/audit/log`): the newest merges, links, unlinks, dismissals, corrections and resets with who
-  did it, filter by action, paged.
+  players"; Link or Dismiss; paged, 20 pairs a page. **Audit log** (`/audit/log`): the newest merges, merge undos, links, unlinks, dismissals, corrections and resets with who
+  did it, filter by action, paged; "Undo" on a merge made in the last 30 days.
 - **Delete my account** in the settings: a dialog asks for the own email and the password. Auth blocks the
   account at once and removes it after 30 days; the page ends the session and goes to the landing page.
 - **Build version** in the settings: commit and build time of Frontend, Auth and Core, each hash links to its