fix: Remove label constraint from parameter seeding

[anth-volk]

Summary

• Removes the constraint that filtered out parameters without labels during database seeding
• Previously, ~91% of US parameters were excluded because bracket parameters (e.g., [0].rate) and breakdown parameters (e.g., .SINGLE, .CA) don't have explicit labels in their YAML files
• Now all parameters are seeded, deduplicated by name

Changes

1. scripts/seed.py: Removed p.label is None check from parameter filtering loop
2. test_fixtures/fixtures_parameters.py: Made label parameter optional in create_parameter()
3. tests/test_parameters.py: Added two tests verifying parameters without labels work correctly

Test plan

• Run pytest tests/test_parameters.py - all 11 tests pass
• Re-seed database and verify US parameters now include bracket/breakdown params

Fixes #58

Will be accompanied by a PR in .py to automatically assign labels to brackets and breakdown parameters and a PR in -us to apply labels to unlabelled parameters.

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