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

internal/handlers/target_retries.go

@@ -90,13 +90,14 @@ func retriesErrorMessage(err error) string {
 		", or 0 for fire-and-forget"
 }
 
-// targetMaxRetries reads and validates max_retries from a target form
+// targetMaxRetries reads and validates max_retries from a target edit
 // submission, answering the request with a 400 and reporting false
 // when the value is set but invalid.
 //
-// Both the create and the edit path go through here, so the two
-// cannot come to disagree about what a valid retry count is. The
-// wording matches the timeout control on the same submission.
+// It and the create path (newTarget) both use parseMaxRetries and
+// retriesErrorMessage, so the two cannot come to disagree about what a
+// valid retry count is. The wording matches the timeout control on
+// the same submission.
 func targetMaxRetries(
 	w http.ResponseWriter,
 	r *http.Request,