Reset channel resume points on undo

--undo only cleared the reactions log, so a subsequent run kept
diffing forward from each channel's last-seen message instead of
honoring days_back, silently skipping everything already undone.
Undo now also clears channel_scan_state so the next run does a
full fresh backlog scan.

bot/undo.py

@@ -43,4 +43,9 @@ async def undo_all_reactions(client: discord.Client, storage: Storage, display:
 
         await asyncio.sleep(human_jitter())
 
-    log.info("Undo complete: removed %d/%d reaction(s)", removed, total)
+    cleared = await asyncio.to_thread(storage.clear_scan_state)
+    log.info(
+        "Undo complete: removed %d/%d reaction(s), cleared %d channel resume point(s) — "
+        "the next normal run will do a full fresh backlog scan.",
+        removed, total, cleared,
+    )