Refactor subscriber queues to bounded tokio mpsc channels with batching and separate QoS1 inflight tracking

[bravo1goingdark]

Motivation

• Reduce contention on per-subscriber state by moving the hot-path queue off a Mutex<VecDeque> and into a bounded channel implementation.
• Reduce per-message locking by batching enqueues in the broker publish path.
• Make QoS1 inflight tracking cheaper on the hot-path by storing inflight state in a separate concurrent map.
• Expose configurable backpressure behavior (drop, block, shed) for full per-subscriber queues.

Description

• Replaced SubscriberQueue internal Mutex<VecDeque<...>> with a bounded tokio::mpsc channel and added enqueue_batch/send_entry helpers to support batching and different backpressure policies via a new BackpressurePolicy enum and BrokerConfig::per_subscriber_backpressure.
• Moved QoS1 inflight tracking out of the queue into SubscriberInflight (a Mutex<HashMap<DeliveryTag, QueueEntry>>) and changed dequeue/ack/maintenance logic to use this separate inflight map.
• Implemented batching in Broker::publish_with_wal_id by collecting subscriber references and calling enqueue_batch to reduce per-message locking and cloning overhead.
• Added QueueEntry::new constructor and helper methods: SubscriberQueue::requeue, SubscriberInflight::maintenance_tick, SubscriberInflight::insert/ack/len and updated maintenance_tick to use the new inflight-based retry/requeue flow.
• Updated call sites and configs across the repo to provide the new backpressure setting (tests, bench, daemon): bench/src/main.rs, blipmqd/src/main.rs, tests/daemon_integration.rs, and tests/chaos_recovery.rs.

Testing

• Ran cargo fmt to format modified files; formatting succeeded.
• No automated unit/integration tests (cargo test) were executed as part of this change.

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Codex Task