Refactored repository and database to improve integrity and clarity

Specifically:

• Added a unique constraints on
  • capabilities for (facility_id, name)
  • target_overrides for (spectral_spec_name, field_source_name, capability_request_id)
  • target_override_values for (target_override_id, name)
  • scans for (position_in_list, observation_specification_id)
• Updated add_entity to raise a ValueError with a nice message instead of an IntegrityError
  • Got rid of some 500 errors in the API as a result
• Added Sphinx-compliant docstrings to ORM repo helpers
• Refactored ORM repo helpers' type hints and parameter names to be more flexible and descriptive