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_create_query_test.go

@@ -176,7 +176,7 @@ func TestHandleTargetCreate_BodyURLStillCreatesTheTarget(t *testing.T) {
 
 	targets := targetsForWebhook(t, env.db, webhook.ID)
 	require.Len(t, targets, 1)
-	assert.Contains(t, targets[0].Config, targetSecretSegments)
+	assert.Equal(t, targetSecretURL, targets[0].URL)
 
 	// The body carried the credential, so the access log must still
 	// not have it: the log records the request line only.