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/delivery/queue_depth_gormlog_test.go

@@ -102,7 +102,7 @@ func TestQueueDepthSample_LogsNoBoundValue(t *testing.T) {
 
 	iCreateTarget(t, mainDB.DB(), targetID, webhookID,
 		"queue-depth-gormlog-target", database.TargetTypeHTTP,
-		iHTTPConfig("https://example.com/hook"), 3,
+		"https://example.com/hook", 3,
 	)
 
 	event := iSeedEvent(