fix(api): run native document conversion off the Node.js event loop - #4433

Problem

convert_document_to_markdown in apps/api/native/src/document.rs was a synchronous #[napi] fn, so the await at its call site in the document engine was a no-op. Every DOCX/ODT/RTF/XLSX/PPTX/EPUB conversion ran directly on the Node.js event loop of whichever pod executed the scrape — including API app pods that run sync scrapes in-process — freezing every unrelated in-flight request for the duration of the conversion.

This is the same bug class fixed for PDFs in e33e1f6 (processPdf/detectPdf).

Fix

• Convert convert_document_to_markdown to pub async fn that offloads the CPU-bound conversion to tokio's blocking thread pool via tokio::task::spawn_blocking, mirroring the pattern in apps/api/native/src/pdf.rs. The binding now takes an owned Uint8Array and returns Promise<string>.
• The engine call site already awaited, so it needed no change; the direct calls in the document-converter snips test are now awaited.

Tests

• New regression test apps/api/src/scraper/scrapeURL/engines/document/__tests__/nativeBinding.test.ts, modeled on the PDF one: generates a ~20MB RTF (no fixtures) whose conversion takes ~1s of CPU and asserts max event-loop lag stays under 250ms. With the old synchronous binding, lag ≈ the full conversion time; with this fix it measures ~2ms.
• Existing document-converter snips tests pass against the async binding (happy paths for DOCX/ODT/RTF/XLSX/CSV and the failure path for unrecognized bytes).

🤖 Generated with Claude Code

Summary by cubic

Makes native document conversion (DOCX/ODT/RTF/XLSX/PPTX/EPUB) run off the Node.js event loop so large conversions no longer freeze unrelated in-flight requests.

Bug Fixes

• The old binding was synchronous, so the await at the engine call site was a no-op and the full conversion blocked the event loop.
• Conversion now offloads to tokio's blocking thread pool via spawn_blocking, matching the PDF processPdf/detectPdf fix.
• The binding takes an owned Uint8Array and returns Promise<string>; the engine call site already awaited and needed no change.
• Adds a regression test that generates a ~20MB RTF and asserts event-loop lag stays under 250ms.

^{Written for commit ce2be95. Summary will update on new commits.}