[TEST] Missing tests for process_webhook_task error path

[arumes31]

This PR addresses missing test coverage for the error-handling logic in process_webhook_task. It ensures that:

• Tasks are correctly moved to the Dead Letter Queue (DLQ) in the database (status 'dlq') when retries are exhausted.
• Transient failures trigger the expected Celery retry mechanism.
• Background log maintenance (cleanup_logs) is verified to respect both default and custom retention policies.

Additionally, global Redis mocking was added to the test suite to improve stability and prevent connection errors in non-Redis environments. It also fixes an issue where some tests failed because the _app variable in tasks.py wasn't correctly initialized/injected during test execution.

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PR created automatically by Jules for task 17351356234223185930 started by @arumes31

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