feat(auth): enhance authentication system with login attempt tracking and configuration options

- Added a new `LoginAttempt` model to track login attempts, including rate limiting and lockout functionality.
- Introduced environment variables for configuring login rate limits and maximum failures.
- Updated the authentication middleware to handle login attempts and enforce rate limits.
- Enhanced the user model with indexing for username and email for improved lookup performance.
- Modified the `.env.example` file to include new optional authentication settings.
- Updated integration tests to cover new login attempt features and authentication state management.

backend/.env.example

@@ -3,6 +3,8 @@ PORT=8000
 NODE_ENV=production
 DATABASE_URL=file:/app/prisma/dev.db
 FRONTEND_URL=http://localhost:6767
+# Optional auth settings
+AUTH_ENABLED=true
 # Optional auth cookie settings: lax | strict | none
 AUTH_COOKIE_SAMESITE=lax
 # Optional auth bootstrap (creates initial admin)
@@ -11,3 +13,6 @@ AUTH_EMAIL=admin@example.com
 # If not set, a random password is generated and logged
 AUTH_PASSWORD=
 AUTH_MIN_PASSWORD_LENGTH=7
+# Optional login throttling
+LOGIN_RATE_LIMIT_MAX=10
+LOGIN_MAX_FAILURES=5