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/engine_lifecycle_test.go

@@ -309,13 +309,12 @@ func seedHeldDelivery(
 	))
 	t.Cleanup(ts.Close)
 
-	cfg := iHTTPConfig(ts.URL)
 	targetID := uuid.New().String()
 
 	iCreateWebhook(t, s.MainDB, s.WebhookID, "hook")
 	iCreateTarget(
 		t, s.MainDB, targetID, s.WebhookID, "held",
-		database.TargetTypeHTTP, cfg, maxAttempts,
+		database.TargetTypeHTTP, ts.URL, maxAttempts,
 	)
 
 	event := iSeedEvent(t, s.WebhookDB, s.WebhookID, `{"held":true}`)
@@ -324,7 +323,7 @@ func seedHeldDelivery(
 	bodyStr := event.Body
 
 	return iTask(
-		d, event, s.WebhookID, targetID, "held", cfg,
+		d, event, s.WebhookID, targetID, "held", ts.URL,
 		maxAttempts, attemptNum, &bodyStr,
 	), arrived
 }