- Added security headers middleware to enforce best practices (e.g., XSS and clickjacking prevention, CSP, HSTS in production).
- Updated `User` model schema: refined field constraints and switched `preferences` to `JSONB` for PostgreSQL compatibility.
- Introduced tests to validate security headers across endpoints and error responses.
- Ensured headers like `X-Frame-Options`, `X-Content-Type-Options`, and `Permissions-Policy` are correctly configured.
This commit introduces a suite of tests for user models, schemas, CRUD operations, and authentication services. It also adds utilities for in-memory database setup to support these tests and updates environment settings for consistency.
