Pure code movement. Every declaration of
internal/handlers/source_management.go moves unchanged into one of:
webhook_list.go, webhook_create.go, webhook_detail.go, webhook_edit.go
and webhook_delete.go for the webhook pages; event_log.go for the event
log; entrypoint.go for the entrypoint handlers; target_create.go,
target_delete.go and target_toggle.go for the target handlers; and
shared.go for the helpers several of them use. Only each file's package
line and imports are new. The README and a middleware comment that
named the removed file now name the new ones.
Model: opus-5-5
