Store a target's settings as columns (closes #543)

A target's settings move from the per-type JSON string in
targets.config to columns: url, headers, timeout_seconds,
forward_query, archive_expiry and archive_rotation. A column a type
does not use stays empty. headers holds the header map as JSON text
through GORM's JSON serializer, and archive_expiry holds a
time.Duration, 0 meaning never, so nothing that reads a target parses
it. The forms check each setting once, with the same refusals and
messages, and the edit form is filled from the columns, so the
per-type config structs, their parsers and the "could not be read"
branches are gone. A delivery task carries the six settings in place
of the JSON string.

Model: opus-5-5

internal/handlers/webhook_detail.go

@@ -109,7 +109,7 @@ func (h *Handlers) renderSourceDetail(
 	data := map[string]any{
 		tmplKeyWebhook: &webhook,
 		// Targets are projected to a display-safe view: a
-		// target's stored config blob holds a credential, and it
+		// target's URL and headers hold a credential, and they
 		// must never reach a template.
 		"Entrypoints":     entrypointViews,
 		"Targets":         h.targetRows(&webhook, targets),