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/target_list.go

@@ -243,10 +243,7 @@ func readTargetDeliveries(
 func (h *Handlers) archiveFileView(
 	webhook *database.Webhook, target *database.Target, now time.Time,
 ) *ArchiveFileView {
-	current, err := archive.PathAt(h.dbMgr, webhook, target, now)
-	if err != nil {
-		return h.archiveUnreadable(&ArchiveFileView{}, target, err)
-	}
+	current := archive.PathAt(h.dbMgr, webhook, target, now)
 
 	view := &ArchiveFileView{Name: filepath.Base(current)}