Users and future authentication logic

[DiazzzDev]

This pull request introduces the foundational user and authentication domain
for BFlow, designed with scalability, security, and future OAuth integration
in mind.

The changes establish a clear separation between user identity, roles, and
authentication providers, enabling flexible authentication strategies without
tight coupling to a single auth mechanism.

What’s included

• Core User entity and status handling
• Role-based access model (users, roles, user_roles)
• Authentication account abstraction supporting local and external providers
• Database structure prepared for future OAuth providers (Google in this case)

Why this change

Authentication and authorization are core concerns in BFlow. Defining these
concepts early allows other modules (wallets, expenses, budgets) to rely on a
stable and secure identity layer.

This design avoids future refactors when introducing OAuth or multiple
authentication providers.

Impact

• No business logic changes
• No runtime behavior changes
• Structural and domain-level foundation only