Follow-up to cherry-picked PR #6592:
- Extract _webhook_url property to deduplicate URL construction
- Add _find_registered_webhooks() helper for reuse
- Crash resilience: check for existing registration before POSTing
(handles restart after unclean shutdown without creating duplicates)
- Accept 200-299 status range (not just 200) for webhook creation
- Unregister removes ALL matching registrations (cleans up orphaned dupes)
- Add 17 tests covering register/unregister/find/edge cases