Bound the web-push delivery queue (max_queue, not the ignored queue_size)

Concurrent::ThreadPoolExecutor takes :max_queue; :queue_size was silently
ignored, leaving the delivery backlog unbounded (max_queue: 0). A flood of
valid push subscriptions could accumulate queued deliveries without limit --
more acute now that each delivery task also resolves DNS. Using max_queue: 10000
activates the intended cap; overflow raises RejectedExecutionError under the
default :abort policy, which deliver_later already rescues (push is best-effort,
retried on the next message).

lib/web_push/pool.rb

@@ -3,7 +3,12 @@ class WebPush::Pool
   attr_reader :delivery_pool, :invalidation_pool, :connection, :invalid_subscription_handler
 
   def initialize(invalid_subscription_handler:)
-    @delivery_pool = Concurrent::ThreadPoolExecutor.new(max_threads: 50, queue_size: 10000)
+    # max_queue (not queue_size, which ThreadPoolExecutor silently ignores) caps
+    # the delivery backlog: an unbounded queue lets a flood of subscriptions
+    # accumulate without limit, more so now that each task also resolves DNS.
+    # Overflow raises RejectedExecutionError under the default :abort policy,
+    # which deliver_later rescues -- push is best-effort and retried next message.
+    @delivery_pool = Concurrent::ThreadPoolExecutor.new(max_threads: 50, max_queue: 10000)
     @invalidation_pool = Concurrent::FixedThreadPool.new(1)
     @connection = Net::HTTP::Persistent.new(name: "web_push", pool_size: 150)
     @invalid_subscription_handler = invalid_subscription_handler