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/source_logs_deleted_target_test.go

@@ -161,7 +161,7 @@ func TestHandleSourceLogs_MasksDeletedTargetConfig(t *testing.T) {
 	tgt := seedConfiguredTarget(
 		t, db, wh.ID,
 		database.TargetTypeSlack,
-		`{"webhookUrl":"`+slackWebhookURL+`"}`,
+		database.Target{URL: slackWebhookURL},
 	)
 
 	seedDeliveredEvent(t, dbMgr, wh.ID, tgt.ID)
@@ -175,7 +175,6 @@ func TestHandleSourceLogs_MasksDeletedTargetConfig(t *testing.T) {
 	assert.NotContains(
 		t, body, "XXXXXXXXXXXXXXXXXXXXXXXX",
 	)
-	assert.NotContains(t, body, "webhookUrl")
 
 	// The name is there; only the credential is not.
 	assert.Contains(t, body, tgt.Name+deletedMarker)