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. 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; tests check that each copy
into a task, and back out of one, keeps all six.

Model: opus-5-5

internal/server/routes.go

@@ -311,7 +311,7 @@ func (s *Server) setupSourceRoutes() {
 		r.Post("/targets", s.h.HandleTargetCreate())
 		// The edit form is the one page that renders a target's
 		// destination URL and header values in full; see
-		// delivery.TargetConfigForm. It belongs to this group for
+		// Handlers.HandleTargetEdit. It belongs to this group for
 		// its RequireAuth and NoCache, which are what keep that
 		// exception from reaching an unauthenticated request or a
 		// shared cache.