Two-phase backlog scan with self-healing found-count tracking

Split each channel's backlog scan into a find phase (settles the
found count first) and a react phase, so the progress display no
longer grows both numbers in lockstep. Found messages are logged in
a dedicated, message_id-deduped table and recovered across restarts
independent of the resume checkpoint, so nothing found is silently
abandoned if the script stops mid-react.

Also drops the separately-maintained messages_found counter, which
proved prone to drift under repeated interruptions/rescans, in favor
of computing it live from the dedup table - self-healing regardless
of how many times a channel gets rescanned.

bot/undo.py

@@ -44,6 +44,9 @@ async def undo_all_reactions(client: discord.Client, storage: Storage, display:
         await asyncio.sleep(human_jitter())
 
     cleared = await asyncio.to_thread(storage.clear_scan_state)
+    await asyncio.to_thread(storage.reset_message_counts)
+    if display is not None:
+        await display.refresh()
     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.",