Add user session tracking, schemas, utilities, and per-device session management

- Introduced `user_sessions` table with support for per-device authentication sessions.
- Added `UserSession` model, including fields for device metadata, IP, and session state.
- Created schemas (`SessionBase`, `SessionCreate`, `SessionResponse`) to manage session data and responses.
- Implemented utilities for extracting and parsing device information from HTTP requests.
- Added Alembic migration to define `user_sessions` table with indexes for performance and cleanup.

backend/app/models/__init__.py

@@ -6,9 +6,11 @@ Imports all models to ensure they're registered with SQLAlchemy.
 from app.core.database import Base
 from .base import TimestampMixin, UUIDMixin
 
-# Import user model
+# Import models
 from .user import User
+from .user_session import UserSession
+
 __all__ = [
     'Base', 'TimestampMixin', 'UUIDMixin',
-    'User',
+    'User', 'UserSession',
 ]