chore(sticker): drop the retired per-user pack records at startup

The module stores nothing: /addsticker takes its pack from STICKER_PACK_NAME
and the set owner from OWNER_ID, and the factory ignores the collection it is
handed. Everything still in the sticker collection is therefore unreachable by
any code path — pack documents keyed by owner ID, "slug:" name reservations and
"pending-delete:" confirmations, all orphaned when the per-user commands were
removed.

InitStore lists and deletes them once per database, guarded by a systemstate
marker in the same shape as the stock and stats migrations. It aborts without
writing the marker so a partial run retries on the next boot, and deletes are
idempotent. A collection that is already empty is the normal case on a fresh
deploy and on the memory backend.

This permanently removes data. Back up the sticker collection before the first
deploy that carries it.

docs/sticker-packs.md

@@ -4,6 +4,15 @@
 bot contributes to. It is the whole of the `sticker` module — one command, no
 storage, and no per-user packs.
 
+**The module stores nothing.** Its factory ignores the collection it is handed:
+the pack comes from the environment and the set owner from `OWNER_ID`, so there
+is nothing per-user to key. A one-time startup cleanup
+(`migration:sticker-drop-legacy-packs-v1`) removes the records the retired
+per-user pack commands left in the `sticker` collection — pack documents keyed
+by owner ID, `slug:` name reservations, and `pending-delete:` confirmations.
+It is marker-guarded, so it scans once per database and never touches anything
+written afterwards.
+
 | Command | Parameters | Reply to | What it does |
 |---|---|---|---|
 | `/addsticker` | `[emoji...]` | sticker, photo, or image document | Adds it to the shared pack and replies with the link |