Add spinner/countdown-bar to stats panel; backfill on widened days_back

Stats panel: a persistent spinner marks whichever target is currently
being scanned/reacted to, and jittered waits show a live-ticking
countdown bar instead of a static "waiting Xs" line. Both the status
line and countdown row are always rendered (blank when idle) so their
appearing/disappearing no longer shifts the table's height.

Backlog: channel_scan_state now tracks oldest_covered_at alongside
newest_id_seen. If days_back is widened after a channel already has a
resume checkpoint, the forward-only diff would never look back far
enough to notice — now the newly-exposed gap gets backfilled first.

bot/reactor.py

@@ -49,10 +49,11 @@ async def react_to_message(
     chan_label = getattr(message.channel, "name", None) or target.name
     delay = human_jitter()
     if display is not None:
-        display.set_status(f"Waiting {delay:.1f}s before reacting in {chan_label}...")
+        display.start_countdown(delay, f"reacting in {chan_label}")
     await asyncio.sleep(delay)
 
     if display is not None:
+        display.clear_countdown()
         display.set_status(f"Sending reaction in {chan_label}...")
     try:
         await message.add_reaction(emoji)