Route the delivery tests' gorm.Open through gormlog (closes #462)

The six gorm.Open calls in the internal/delivery tests passed a bare
gorm.Config, which installs GORM's default logger; they now pass
gormlog.New over a logger that discards, as every production call does,
so the unfiltered form is no longer in the tree to be copied.

The README and the ParamsFilter comment said (*gorm.DB).Scan had one
test-only caller; there are more. Both now say only tests call it, with
fixture data.

Model: opus-5-5

internal/delivery/engine_test.go

@@ -25,6 +25,7 @@ import (
 	_ "modernc.org/sqlite"
 	"sneak.berlin/go/webhooker/internal/database"
 	"sneak.berlin/go/webhooker/internal/delivery"
+	"sneak.berlin/go/webhooker/internal/gormlog"
 	"sneak.berlin/go/webhooker/internal/metrics"
 )
 
@@ -49,7 +50,8 @@ func testWebhookDB(t *testing.T) *gorm.DB {
 	t.Cleanup(func() { _ = sqlDB.Close() })
 
 	db, err := gorm.Open(
-		sqlite.Dialector{Conn: sqlDB}, &gorm.Config{},
+		sqlite.Dialector{Conn: sqlDB},
+		&gorm.Config{Logger: gormlog.New(slog.New(slog.DiscardHandler))},
 	)
 	require.NoError(t, err)