Introduce the `EventManager` model to manage event roles and permissions. Roles include `OWNER`, `ADMIN`, `MODERATOR`, and `VIEWER`, each with specific default permissions. The model supports explicit permissions, relationships, constraints, and a helper method for initializing event owners.
Introduces models for events and their themes, including fields for event details, location, timing, and settings. The `EventTheme` model supports theme customization with color palettes and fonts, while establishing relationships with the `Event` model. These additions prepare the database for event-related functionality.
Created reusable `Base`, `UUIDMixin`, and `TimestampMixin` in `base.py`. Added a `User` model with attributes, relationships, and database configurations in `user.py` to establish a foundation for user-related features.
