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(