Add .env-example file and update configuration for environment variables

- Created .env-example for environment variable setup
- Updated README to reflect changes in .env file creation
- Modified config.py to include new environment variables and DATABASE_URL property
- Enhanced database.py with connect and disconnect functions
- Updated main.py to manage database connection lifecycle
- Adjusted compose.yaml for consistent environment variable usage

app/config.py

@@ -1,6 +1,23 @@
 from pydantic_settings import BaseSettings
+from pydantic import computed_field
 
 class Settings(BaseSettings):
-    DATABASE_URL: str = "${DATABASE_URL}"
+    POSTGRES_USER: str
+    POSTGRES_PASSWORD: str
+    POSTGRES_DB: str
+    API_PORT: int = 8000
+    DEBUG: bool = False
+
+    @computed_field
+    @property
+    def DATABASE_URL(self) -> str:
+        return (
+            f"postgresql+asyncpg://"
+            f"{self.POSTGRES_USER}:{self.POSTGRES_PASSWORD}"
+            f"@db/{self.POSTGRES_DB}"
+        )
+
+    class Config:
+        env_file = ".env"
 
 settings = Settings()