Targets section: one Add, then a type and Next, then its fields (closes #370)

The webhook page's targets section lists only its targets until Add is
clicked. Add shows a choice of target type with Next; Next shows only
that type's fields, with Save and Cancel. The `database` and `log`
types have no URL field, and the `slack` form gains max retries.

A refused target now shows the webhook page again with the form open on
its type, the values entered and the reason, instead of a bare text
page. Target validation returns that message rather than writing the
response; newTarget validates a whole new target for reuse by the
new-webhook page. The edit page still answers a refusal in plain text.

Model: opus-5-5

README.md

@@ -1344,10 +1344,13 @@ markup. The CSP build runs no expressions, so every Alpine directive in
 `x-data="{ open: false }"` or `@click="open = !open"`.
 
 A browser test in `internal/server` loads the webhook page and the event log
-under the real policy and checks that: both add forms stay hidden until Add is
-clicked; choosing Slack in the add target form leaves the HTTP fields out of
-what it submits, also after leaving the page and going back to it, when the
-browser restores the choice; the Copy button beside an entrypoint URL reads
+under the real policy and checks that: the add entrypoint form stays hidden
+until Add is clicked; for every target type, the targets section's Add shows
+only a choice of type with Next and Cancel, Next shows only that type's fields
+(no url field for `database` or `log`), Cancel at either step closes the form,
+and saving adds the target; a refused target comes back with its form open, the
+values entered and the reason, and after Cancel the next Add starts with an
+empty form and no reason; the Copy button beside an entrypoint URL reads
 "Copied" once clicked; an entrypoint's Edit button shows its edit form in place
 of its description and hides until the form closes, Cancel hides the form and
 drops what was typed, as does leaving the page and going back to it, and Save