feat: add artifact index storage interface

[ch-liuzhide]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 90.76923% with 6 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/whiskerrag_types/model/space.py	63.63%	2 Missing and 2 partials ⚠️
src/whiskerrag_types/model/artifact_index.py	95.23%	1 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
...rrag_types/interface/db_engine_plugin_interface.py	91.66% <100.00%> (+0.69%)	⬆️
src/whiskerrag_types/model/artifact_index.py	95.23% <95.23%> (ø)
src/whiskerrag_types/model/space.py	87.23% <63.63%> (-7.37%)	⬇️

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