feat: add defaultable field support with companion struct generation

Introduces support for `#[georm(defaultable)]` attribute on entity
fields. When fields are marked as defaultable, generates companion
`<Entity>Default` structs where defaultable fields become `Option<T>`,
enabling easier entity creation when some fields have database defaults
or are auto-generated.

Key features:
- Generates `<Entity>Default` structs with optional defaultable fields
- Implements `Defaultable<Id, Entity>` trait with async `create` method
- Validates that `Option<T>` fields cannot be marked as defaultable
- Preserves field visibility in generated companion structs
- Only generates companion struct when defaultable fields are present

docker/compose.dev.yml

@@ -4,9 +4,9 @@ services:
     restart: unless-stopped
     container_name: georm-backend-db
     environment:
-      POSTGRES_PASSWORD: ${DB_PASSWORD}
-      POSTGRES_USER: ${DB_USER}
-      POSTGRES_DB: ${DB_NAME}
+      POSTGRES_PASSWORD: georm
+      POSTGRES_USER: georm
+      POSTGRES_DB: georm
     ports:
       - 127.0.0.1:5432:5432
     volumes: