feat: implement filament and spool preset services with CRUD operations

- Add `FilamentService` for managing filament inventory, including creation, retrieval, updating, and deletion of filaments.
- Introduce `SpoolPresetService` for handling spool presets, allowing users to create, update, and delete their custom presets.
- Create domain models for `Filament` and `SpoolPreset` with necessary fields and methods.
- Define repository interfaces for filament and spool preset persistence.
- Implement application configuration management from environment variables.
- Set up error handling with a centralized `AppError` type.
- Build the Axum router with public and protected routes for user authentication and resource management.

backend/.env.example

@@ -0,0 +1,18 @@
+# Banco de dados
+DATABASE_URL=postgresql://meowspool:meowspool@localhost:5432/meowspool
+
+# JWT
+JWT_SECRET=troque_por_um_secret_forte_em_producao_minimo_32_chars
+JWT_EXPIRY_SECS=3600
+JWT_REFRESH_EXPIRY_SECS=2592000
+
+# OAuth Google
+GOOGLE_CLIENT_ID=seu_google_client_id.apps.googleusercontent.com
+GOOGLE_CLIENT_SECRET=seu_google_client_secret
+
+# Servidor
+HOST=0.0.0.0
+PORT=8080
+
+# Ambiente (development | production)
+APP_ENV=development