Feat/webhook delivery stats

[amandapada]

Summary
Closes #184

Adds GET /api/webhooks/{id}/stats/ — a cached endpoint returning delivery health statistics for a webhook subscription.

Changes

models.py

• Added duration_ms (FloatField, nullable) to WebhookDeliveryLog to track HTTP request duration per attempt

tasks.py

• Updated _log_delivery_attempt to accept and store duration_ms
• All 4 call sites inside dispatch_webhook now pass (time.monotonic() - _start) * 1000 as duration_ms

views.py

• Added stats action to WebhookSubscriptionViewSet
• Returns: total_deliveries, successful, failed, success_rate, avg_latency_ms, last_delivery_at
• Response cached for 60s per subscription

migrations/

• New migration for duration_ms field on WebhookDeliveryLog

tests/test_views.py

• test_webhook_stats_empty — zero state returns correct defaults
• test_webhook_stats — correct aggregation across success/failure logs
• test_webhook_stats_cached — verifies 60s cache is set after first request
• test_webhook_stats_404_for_missing_webhook — non-existent ID returns 404

Acceptance Criteria

• Endpoint returns all stats
• Success rate calculated correctly
• Latency accurate (recorded at actual dispatch time via time.monotonic())
• Cached for 60s

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[DokaIzk]

@amandapada please resolve conflicts