feat(auth): setup better-auth backend with email and oauth support

[Nirbhay21]

Summary

• Integrated Better Auth on the backend with email/password and OAuth support
• Introduced auth database schema with sessions, accounts, users, and verification
• Added signup metadata validation aligned with frontend contract
• Enabled Row Level Security (RLS) and policies for auth tables

Scope

Backend authentication setup only:

• Better Auth server configuration
• Auth database schema, migrations, and policies
• Email/password and OAuth (Google, GitHub) providers
• Signup metadata validation (role, terms acceptance)

Context

SmartAssess frontend already relies on Better Auth and expects:

• Explicit user metadata (role, termsAccepted)
• OAuth signup support with additional data
• Session-based authentication

This PR establishes the backend foundation required to support that contract.

What

• Added Better Auth server configuration with Drizzle adapter
• Introduced auth schema with:
  • user, session, account, and verification tables
  • Proper relations and indexes
• Implemented signup metadata validation and enforcement:
  • Role validation
  • Mandatory terms acceptance with versioning
• Enabled Row Level Security (RLS) and defined policies for all auth tables
• Added OAuth providers (Google, GitHub) with secure callbacks
• Centralized environment variables (.env) validation using Zod
• Cleaned up legacy schema and removed unused demo tables
• Updated Drizzle config and migration structure

Testing

• Email signup tested via frontend
• OAuth signup tested for Google and GitHub
• User metadata (role, termsAccepted, timestamps) verified in database
• Session creation and persistence validated