Refactor authentication services to async password handling; optimize bulk operations and queries

- Updated `verify_password` and `get_password_hash` to their async counterparts to prevent event loop blocking.
- Replaced N+1 query patterns in `admin.py` and `session_async.py` with optimized bulk operations for improved performance.
- Enhanced `user_async.py` with bulk update and soft delete methods for efficient user management.
- Added eager loading support in CRUD operations to prevent N+1 query issues.
- Updated test cases with stronger password examples for better security representation.

backend/tests/schemas/test_user_schemas.py

@@ -111,7 +111,7 @@ class TestPhoneNumberValidation:
             email="test@example.com",
             first_name="Test",
             last_name="User",
-            password="Password123",
+            password="Password123!",
             phone_number="+41791234567"
         )
         assert user.phone_number == "+41791234567"
@@ -122,6 +122,6 @@ class TestPhoneNumberValidation:
                 email="test@example.com",
                 first_name="Test",
                 last_name="User",
-                password="Password123",
+                password="Password123!",
                 phone_number="invalid-number"
             )