[BUG] Webhook dead-letter queue grows unbounded — add pruning job

[ritik4ever]

Category: backend
Project area: backend/src/services/

Description:
Dead-letter entries accumulate in webhook_dead_letters table without any pruning. Under sustained webhook failures this table can grow to millions of rows.

Acceptance Criteria:

• Scheduled job prunes entries older than 30 days.
• Prune interval configurable via env var.
• Admin endpoint to manually clear dead-letter queue.