Feature/webhook delivery metrics endpoint

[godamongstmen897]

Summary

Closes #584. Establishes an authenticated REST API metrics endpoint (/api/webhooks/deliveries/metrics/) providing deep visibility into webhook ingestion pipeline health, latency parameters, and delivery trends.

What Changed

• Metrics Aggregation Engine: Authored modular data view query logic in views.py calculating rolling historical delivery success rates, volume totals, and average response intervals.
• Granular Query Scopes: Implemented support for optional subscription_id mapping and dynamic time-window range filtering bounds.
• Dual Routing Integration: Registered the endpoints cleanly across both primary application profiles (urls.py) and explicit testing execution frames (urls_test.py).
• Suite Verification: Created test_webhook_metrics.py verifying request identity evaluation parameters and calculation correctness using native repository DRF testing fixtures.

Testing & Validation

• Confirmed mathematical metrics formatting evaluates flawlessly via local pytest environments.
• Verified parameter filters intercept target intervals accurately without database leak anomalies or authentication bypass gaps.

Closes #584

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