feat(05-01): restructure fonoteka plugins into Winter directories

- Move models, classes, controllers, middleware, and updates into subpackages
- Add self-registering models/updates/classes registries so later plans add files
- Keep the genres route and shipped migration SQL unchanged

classes/registry.go

@@ -0,0 +1,21 @@
+package classes
+
+import "gorm.io/gorm"
+
+var hookRegistrars []func(*gorm.DB) error
+
+// RegisterHook appends a GORM-callback registrant so later plans add files
+// without editing plugin.go Boot.
+func RegisterHook(fn func(*gorm.DB) error) {
+	hookRegistrars = append(hookRegistrars, fn)
+}
+
+// RegisterHooks runs every registered hook against gdb.
+func RegisterHooks(gdb *gorm.DB) error {
+	for _, fn := range hookRegistrars {
+		if err := fn(gdb); err != nil {
+			return err
+		}
+	}
+	return nil
+}