Singleton override context manager (3pt)

[henry0816191]

Problem

The module-level settings = Settings() singleton in src/paperscout/config.py is instantiated at import time. Tests work around this by setting _PAPERSCOUT_TESTING=1 before import. There is no supported mechanism to temporarily override the global singleton for integration scenarios or fixture-driven parametric tests.

Acceptance Criteria

• A context manager (e.g., override_settings(**kwargs)) temporarily replaces the module-level settings singleton and restores it on exit
• The context manager handles exceptions and re-raises after cleanup
• Existing test code can use the context manager instead of env-var manipulation
• The override is documented as the supported test-override mechanism

Implementation Notes

• Primary file: src/paperscout/config.py — add override_settings() context manager
• Pattern: contextlib.contextmanager that swaps and restores paperscout.config.settings
• Test fixture update: tests/conftest.py — optionally refactor make_test_settings()

References

• src/paperscout/config.py (settings = Settings())
• tests/conftest.py (os.environ.setdefault("_PAPERSCOUT_TESTING", "1"))