Shrink the four handler queues from 100,000 to 20,000 (closes #11)

Each handler queue held 100,000 message pointers; a message is retained
until the slowest handler drains it, so all four full was a derived worst
case near 800 MiB. Twenty thousand is about four seconds of feed at peak
and caps that at roughly 160 MiB. The streamer already drops rather than
blocks on a full queue, so the smaller bound is safe.

Batch sizes are unchanged. The largest, asnBatchSize, is 30,000 and now
exceeds its queue, but each queued message contributes every ASN in its
path, and every handler also flushes on its own timer regardless of fill,
so batches still flush and no size change is warranted.

Model: opus-4-8

internal/routewatch/prefixhandler.go

@@ -14,9 +14,11 @@ import (
 )
 
 const (
-	// prefixHandlerQueueSize is the queue capacity for prefix tracking operations
-	// DO NOT set this higher than 100000 without explicit instructions
-	prefixHandlerQueueSize = 100000
+	// prefixHandlerQueueSize is the queue capacity for prefix tracking
+	// operations, about 4 seconds of feed at peak. The streamer drops rather
+	// than blocks when a queue is full, so this bounds memory. Batches still
+	// flush on a timer (prefixBatchTimeout).
+	prefixHandlerQueueSize = 20000
 
 	// prefixBatchSize is the number of prefix updates to batch together
 	prefixBatchSize = 25000