fix(server): return 503 on full Slack queue instead of acking-and-dropping (#356)

[gnanirahulnutakki]

Summary

Fixes #356 — inbound Slack messages were silently dropped when the bounded worker queue was full, yet Slack still received a 200 ack and never retried. Under burst (queue depth > WORKER_QUEUE_CAPACITY = 1024) real messages were lost with a misleading log, and the 2xx told Slack not to redeliver.

Root cause

• crates/server/src/state.rs — AppState::enqueue collapsed mpsc::error::TrySendError::Full and Closed into a single bool (try_send(...).is_ok()), erasing the retryable-vs-terminal distinction.
• crates/server/src/routes.rs — slack_events logged "turn worker is gone; dropping message" and returned StatusCode::OK regardless of why the enqueue failed.

This was asymmetric with the /chat path, which already surfaces a full queue as 503.

Fix

• enqueue now returns a three-way EnqueueOutcome { Accepted, Full, WorkerGone } instead of a bool.
• slack_events acks 200 only on Accepted; on both Full and WorkerGone it returns 503 SERVICE_UNAVAILABLE so Slack redelivers rather than treating the message as durably received. This mirrors the existing /chat QueueFull → 503 behavior.
• The long-poll forwarder (Matrix/Discord/Telegram) is intentionally untouched — those transports have no HTTP redelivery contract and already log-and-drop on Full / break on Closed.

Regression test

state::tests::enqueue_distinguishes_full_queue_from_a_gone_worker fills a capacity-1, undrained queue and asserts:

• first message → Accepted
• second (queue saturated) → Full (retryable, not silently accepted, not WorkerGone)
• after dropping the receiver → WorkerGone

The test exercises the exact distinction the fix introduces; it does not compile against the old bool API.

Verification

All green locally (macOS, toolchain 1.96.1):

• cargo fmt --check
• cargo clippy -p ardur-server --all-targets -- -D warnings
• cargo test -p ardur-server (incl. the new regression test)
• cargo test -p ardur-e2e-tests

Not merged — left for peer review.

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