`POST /admin/hooks` reads `is_system_webhook` out of the config map and
defaults it to false, which creates a forgejo *default* webhook — a
template copied into repos created later — instead of a live
instance-wide one. `GET /admin/hooks` returns only hooks with the flag
set, so `list_hook_urls` could never see what the create had just made:
every controller start listed zero hooks and created another default
webhook (12 in 26h), while the instance-wide push observation the scope
exists for never fired at all.
Send the key on the `Instance` arm only, via a per-scope
`extra_create_config()` so repo and org scopes stay unchanged.
Closes#3807
2b35ffd250