fix(webhook): offload SLA webhook dispatch to Celery task

[LaGodxy]

Summary

Closes #234

Offloads webhook dispatch from the synchronous call path in trigger_sla_violation_webhooks() to the existing dispatch_webhook_delivery Celery task. When a broker is configured (task_always_eager=False), each delivery is enqueued via apply_async instead of blocking the caller. Falls back to synchronous dispatch in eager/single-worker mode or when Celery is unavailable.

Why

The webhook dispatcher called dispatch_delivery() synchronously in a loop, blocking the SLA computation response while outbound HTTP requests completed (10s timeout each). With multiple webhooks, this caused timeout cascades. The existing dispatch_webhook_delivery Celery task was already defined but never wired into the trigger path.

What was built

app/services/webhook_service.py:

File	What it contains
dispatch_delivery()	Added idempotency guard: skips deliveries already in terminal state (SUCCESS, DEAD_LETTER) to prevent duplicate HTTP calls from concurrent task executions
trigger_sla_violation_webhooks()	Detects Celery availability at runtime; enqueues dispatches via dispatch_webhook_delivery.delay() when broker is configured, falls back to synchronous call otherwise

tests/test_webhook_async_dispatch.py: (new)

File	What it contains
TestTriggerSlaViolationAsyncDispatch	5 tests: Celery dispatch when broker configured, sync fallback in eager mode, sync fallback when import fails, no-webhook empty return, multiple webhooks each enqueued
TestDispatchDeliveryIdempotency	4 tests: skips SUCCESS, skips DEAD_LETTER, dispatches PENDING, dispatches RETRYING

Integration changes outside <module>/

No existing files modified beyond app/services/webhook_service.py. The existing dispatch_webhook_delivery task in app/tasks/webhook_tasks.py and retry_pending_webhook_deliveries beat task remain unchanged.

Acceptance criteria coverage

• Webhook dispatch is offloaded to a Celery task (test_celery_dispatches_via_task_when_broker_configured — verifies dispatch_webhook_delivery.delay() is called)
• SLA calculation endpoints return immediately without waiting for webhook delivery (enqueued via .delay(), not called synchronously)
• The Celery task handles retries and dead-letter marking correctly (existing dispatch_webhook_delivery task + retry_pending_webhook_deliveries beat task unchanged)
• A test verifies that dispatching a webhook does not block the calling thread (test_celery_dispatches_via_task_when_broker_configured — dispatch_delivery is NOT called synchronously)
• Fallback to synchronous dispatch when Celery is unavailable (test_fallback_to_sync_when_eager_mode, test_fallback_to_sync_when_celery_import_fails)

Deliberately deferred

• Converting the webhook service to full async/await (out of scope per issue — focus on Celery offload first)
• Converting WebhookDispatchLimiter and CircuitBreaker to async primitives (requires async-compatible lock/semaphore, separate concern)

Test plan

• pytest tests/test_webhook_async_dispatch.py — 9/9 passing (9 new tests)
• pytest tests/test_webhook_breaker.py tests/test_webhook_dispatch_limiter.py tests/test_webhook_ssrf.py — 20/20 passing (no regressions)

Env vars / Notes

No new env vars. Uses existing CELERY_BROKER_URL and CELERY_TASK_ALWAYS_EAGER. When CELERY_TASK_ALWAYS_EAGER=true (default for local dev), behavior is identical to before — dispatches are synchronous. Set CELERY_TASK_ALWAYS_EAGER=false with a valid Redis broker to get async offload.

[usmanimamu17-create]

LGTM. thanks for contributing.