Hard daily cap, no-flood shutdown, ctx-aware poller, double-image fix

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.

config.example.yaml

@@ -20,6 +20,7 @@ posting:
   min_interval_seconds: 300
   burst_cap_count: 3
   burst_cap_window_seconds: 1800
+  daily_cap_total: 5  # hard global cap across ALL channels (rolling 24h); 0 disables
 
 storage:
   db_path: "./data/pete.db"