Fix workstation route latency

[shiny-code-bot]

Summary

• render Work, Folders, Ops, and Completed shells immediately instead of blocking route paint on heavy API payloads
• shrink /api/completed by deriving cleanup totals from promoted-folder rows and truncating oversized failed-encode history details
• replace folder encode candidate count materialization with a SQL aggregate count
• update smoke routing checks to wait for async workstation data after the loading shell appears

Measured locally

• /api/completed: 8.3 MB / ~0.30s before, 36 KB / ~0.14-0.20s after on the live runtime state
• Completed browser fetch: 36 KB / ~195ms after the fix
• dashboard-heavy routes now show an immediate workstation loading shell while their data request resolves

Validation

• browser-tested Work, Folders, Ops, and Completed through the local dev UI
• uv run --with pytest pytest tests/test_tuning_runtime.py -k "completed_page_payload"
• uv run --with pytest pytest tests/test_encode_queue_recovery.py -k "folder_encode_candidate_count or folder_studio_series_context"
• npm --prefix frontend run lint
• npm --prefix frontend run check
• npm --prefix frontend test
• npm --prefix frontend run build
• npm --prefix frontend run smoke:web
• bash scripts/pre-commit-check.sh