New-webhook page: optional HTTP target URL and archive with pruning (closes #373)

The new-webhook page keeps name, description and retention and adds an
HTTP target URL field and an archive checkbox. A filled-in URL creates
an http target named HTTP; a checked archive shows a pruning choice
(never, 1h, 12h, 24h, 30d, 90d, 365d) and creates a database target
named Archive with that expiry. Each target goes through the add
target form's validation, and the webhook, its entrypoint and its
targets are created in one transaction. A refused form comes back with
every value entered, retention included.

Model: opus-5-5

README.md

@@ -1580,6 +1580,13 @@ more entrypoints (receiver URLs) and one or more targets (delivery
 destinations) into a logical unit. A user creates a webhook to set up
 event routing.
 
+The new webhook form can also give the webhook its first targets: an
+optional HTTP target URL creates an `http` target named `HTTP`, and the
+archive checkbox creates a `database` target named `Archive` whose
+`expiry` is the pruning chosen beside it (never, 1h, 12h, 24h, 30d, 90d
+or 365d). Both are validated as on the add target form, and the webhook
+and its targets are created together or not at all.
+
 | Field            | Type    | Description |
 | ---------------- | ------- | ----------- |
 | `id`             | UUID    | Primary key |