Timeline 2.0: optimize timeline feed pipeline

[ymichael]

Summary

• add the timeline feed contract/routes with bounded previews, lazy detail loading, pagination, and large-value extraction for command/tool/diff output
• move timeline pagination into @bb/thread-view, collapse normal server feed generation to events -> TimelineRow -> TimelineFeedRow, and keep the app/CLI compatibility adapter at the render boundary
• split timeline builder/title logic by work-row family, add event taxonomy/default-quiet handling, and remove stale legacy timeline response imports/helpers
• add DB migration/backfill coverage for legacy large event payloads and update app/CLI/server/integration tests for the feed route

Validation

• git diff --check
• pnpm exec turbo run typecheck --filter=@bb/server --filter=@bb/thread-view --filter=@bb/app --filter=@bb/server-contract --filter=@bb/db --filter=@bb/integration-tests --filter=@bb/cli --filter=@bb/sdk
• pnpm exec turbo run test --filter=@bb/server
• pnpm exec turbo run test --filter=@bb/db
• pnpm exec turbo run test --filter=@bb/thread-view
• pnpm exec turbo run test --filter=@bb/app -- --run src/views/thread-detail/useThreadTimelinePages.test.ts src/components/thread/timeline/timeline-auto-expand.test.ts src/hooks/realtime-cache-effects.test.ts src/hooks/cache-owners/cache-owner-registry.test.ts src/hooks/queries/query-helpers.test.ts

Notes

• The old full timeline route is not restored; callers are migrated to the feed route.
• The remaining TimelineFeedRow -> TimelineFeedViewRow adapter is client-side render compatibility code, not part of the server query/generation hot path.