Four related fixes after Pete flooded a channel and ignored Ctrl-C:
1. Global daily cap (posting.daily_cap_total, default 5): hard ceiling on
posts across ALL channels in a rolling 24h window. Checked before the
per-channel min-interval and burst-cap.
2. Shutdown no longer flushes the queue. Previous drainAll posted every
remaining item with rate limits disabled — which was literally the
flood. Replaced with dropOnShutdown that clears queues and logs
the count.
3. Poller respects ctx mid-loop. pollOnceWithErr now takes ctx and bails
between items, so Ctrl-C doesn't have to wait for ~30s of network
per pending story before shutdown can complete.
4. Double-image fix. PostStory now reports imageSent; the queue clears
ImageURL before retry so a text-send failure after a successful
image upload doesn't re-post the image.
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