fix: auto-refresh PR status for all active workspaces in sidebar (Refs: beans-xixh)

[hmans]

Summary

• The PR status icons on sidebar workspace cards didn't auto-refresh — they were only updated when worktree filesystem events fired (file changes, creation, deletion)
• Adds a 30-second periodic ticker to the WorktreesChanged subscription resolver that re-fetches PR data for all active worktrees
• The ticker path only emits after PR data is fully populated (via populatePRsAsync), so there's no sidebar flicker

Test plan

• Open the web UI with multiple workspaces that have PRs
• Verify PR status icons appear on sidebar workspace cards
• Push a commit to one of the PR branches and wait ~30 seconds — the check status icon should update without any manual interaction
• Verify worktrees don't disappear/flicker during the periodic refresh