feat(library): add ALL_UNLOCKED sync change for last achievement

A sync that unlocks the last achievement of a game not COMPLETED
adds an ALL_UNLOCKED change. The overview hides it once the game is
COMPLETED, hidden or removed. Refs #21

readme.md

@@ -426,6 +426,8 @@ and `WISHLIST` rows of `sync_change`) and the hand changes since the cursor. Kin
 are left out, and the cursor moves anyway. A job every 10 minutes reports the hand changes alone once a user was quiet for 10
 minutes; it never takes a sync result, which may still wait for its achievements.
 
+`ALL_UNLOCKED`: the sync unlocked the last Steam achievement of a game that is not COMPLETED; Core never sets COMPLETED itself, the user confirms it in the sync details.
+
 ```mermaid
 sequenceDiagram
     participant W as LibrarySyncWorker
@@ -438,7 +440,7 @@ sequenceDiagram
     P-->>W: ADDED, PLAYTIME, ACHIEVEMENTS (trophy gains)
     W->>D: sync_result, sync_change (ADDED, STATUS, WISHLIST, PSN changes)
     W-->>A: LibrarySyncedEvent
-    A->>D: sync_change (ACHIEVEMENTS)
+    A->>D: sync_change (ACHIEVEMENTS, ALL_UNLOCKED)
     A-->>F: LibraryActivityEvent
     F->>D: lock follow_digest, count sync + hand changes
     F->>D: inbox_note per follower (public, not blocked, not deleted)