Update session cleanup logic, pyproject.toml, and Makefile for consistency and improved tooling support

- Replaced `not UserSession.is_active` with `UserSession.is_active == False` in cleanup queries for explicit comparison.
- Added `mypy` overrides for `app.alembic` and external libraries (`starlette`).
- Refactored `Makefile` to use virtual environment binaries for commands like `ruff`, `mypy`, and `pytest`.

backend/pyproject.toml

@@ -139,6 +139,10 @@ plugins = ["pydantic.mypy"]
 module = "alembic.*"
 ignore_errors = true
 
+[[tool.mypy.overrides]]
+module = "app.alembic.*"
+ignore_errors = true
+
 [[tool.mypy.overrides]]
 module = "sqlalchemy.*"
 ignore_missing_imports = true
@@ -171,6 +175,10 @@ ignore_missing_imports = true
 module = "apscheduler.*"
 ignore_missing_imports = true
 
+[[tool.mypy.overrides]]
+module = "starlette.*"
+ignore_missing_imports = true
+
 # ============================================================================
 # Pydantic mypy plugin configuration
 # ============================================================================