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/storage.py

@@ -132,6 +132,16 @@ class Storage:
                 )
             return inserted
 
+    def clear_scan_state(self) -> int:
+        """Wipe all per-channel resume points. The backlog scan only ever
+        consults days_back on a channel's very first scan and otherwise just
+        diffs forward from the last-seen message, so this must be cleared
+        whenever reactions are undone (or days_back is widened) to force a
+        fresh full backlog scan instead of silently skipping everything."""
+        with self._lock, self._conn:
+            cur = self._conn.execute("DELETE FROM channel_scan_state")
+            return cur.rowcount
+
     def get_scan_state(self, channel_id) -> Optional[dict]:
         with self._lock:
             cur = self._conn.execute(

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,
+    )