Add async CRUD base, async database configuration, soft delete for users, and composite indexes

- Introduced `CRUDBaseAsync` for reusable async operations.
- Configured async database connection using SQLAlchemy 2.0 patterns with `asyncpg`.
- Added `deleted_at` column and soft delete functionality to the `User` model, including related Alembic migration.
- Optimized queries by adding composite indexes for common user filtering scenarios.
- Extended tests: added cases for token-based security utilities and user management endpoints.

backend/app/alembic/versions/b76c725fc3cf_add_composite_indexes.py

@@ -0,0 +1,52 @@
+"""add_composite_indexes
+
+Revision ID: b76c725fc3cf
+Revises: 2d0fcec3b06d
+Create Date: 2025-10-30 16:41:33.273135
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'b76c725fc3cf'
+down_revision: Union[str, None] = '2d0fcec3b06d'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Add composite indexes for common query patterns
+
+    # Composite index for filtering active users by role
+    op.create_index(
+        'ix_users_active_superuser',
+        'users',
+        ['is_active', 'is_superuser'],
+        postgresql_where=sa.text('deleted_at IS NULL')
+    )
+
+    # Composite index for sorting active users by creation date
+    op.create_index(
+        'ix_users_active_created',
+        'users',
+        ['is_active', 'created_at'],
+        postgresql_where=sa.text('deleted_at IS NULL')
+    )
+
+    # Composite index for email lookup of non-deleted users
+    op.create_index(
+        'ix_users_email_not_deleted',
+        'users',
+        ['email', 'deleted_at']
+    )
+
+
+def downgrade() -> None:
+    # Remove composite indexes
+    op.drop_index('ix_users_email_not_deleted', table_name='users')
+    op.drop_index('ix_users_active_created', table_name='users')
+    op.drop_index('ix_users_active_superuser', table_name='users')