Redesign spectrum model database architecture

[tlambert03]

This refactoring improves the multiple nullable ForeignKey pattern for spectrum ownership with the following enhancements:

Centralization & DRY:

• Add OWNER_FIELD_CONFIG and OWNER_FIELD_NAMES constants as single source of truth for owner field configuration
• Derive SpectrumForm.lookup from OWNER_FIELD_CONFIG to eliminate duplication
• Add get_owner_field_name() and get_category_for_owner_field() helper methods

Database Integrity:

• Add CheckConstraint to enforce "exactly one owner" at database level
• Auto-populate category field from owner type in save() method
• Better validation error messages

Code Quality:

• Refactor sluglist() to use dynamic field iteration with next()
• Update filter_owner() and find_similar_owners() to use constants
• Add type hints to properties (owner_set, owner)
• Change owner property to @cached_property for performance
• Improve docstrings and add inline documentation

Documentation:

• Document rationale for multiple nullable ForeignKeys vs GenericForeignKey pattern
• Add comments explaining why category field is kept despite being derivable
• Reference Luke Plant's blog post for future maintainers

No breaking changes - full backward compatibility maintained.