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.

internal/storage/queries.go

@@ -182,6 +182,17 @@ func GetLastPostTime(channel string) int64 {
 	return t
 }
 
+// CountAllPostsInWindow counts posts across ALL channels within a time window.
+// Used for the global daily cap.
+func CountAllPostsInWindow(windowStart int64) int {
+	var count int
+	if err := Get().QueryRow(`SELECT COUNT(*) FROM post_log WHERE posted_at >= ?`, windowStart).Scan(&count); err != nil {
+		slog.Error("CountAllPostsInWindow query failed", "err", err)
+		return 1<<31 - 1 // fail closed: huge number prevents posting
+	}
+	return count
+}
+
 // CountPostsInWindow counts posts to a channel within a time window.
 func CountPostsInWindow(channel string, windowStart int64) int {
 	var count int