Add thread environment status snapshots

[ymichael]

Summary

• add server-owned git and pull request status snapshots for thread environments
• project environment status summaries into thread list entries for sidebar consumers
• render sidebar PR/git attention indicators and collapsed child rollups
• add regression coverage for the due snapshot refresh worker writing snapshots and notifying attached threads

Validation

• pnpm exec turbo run typecheck --filter=@bb/domain --filter=@bb/db --filter=@bb/server-contract --filter=@bb/server --filter=@bb/app
• pnpm exec turbo run test --filter=@bb/server -- test/services/environments/environment-status-snapshots.test.ts
• pnpm exec turbo run test --filter=@bb/domain --filter=@bb/db --filter=@bb/server-contract --filter=@bb/server --filter=@bb/app

Note: regenerated DB migration as 0047 after rebasing over current origin/main.