chore: migrate filesystem storage to sqlite db

[typpo]

Large change that moves eval storage from flat json files to sqlite. This unblocks a lot of future development, including things like:

• Unify shared eval store with the rest of eval storage logic
• Easier self-hosting setup/maintenance
• Accounts/access control for hosted version
• Ease of constructing better UIs to enable browsing/comparison of prompts and datasets
• Removes hackiness such as in fix: Only open previous results when necessary (uses lots of memory) #418