Separate OnlyOffice async handling, add office_previews WS flag, framed worker protocol

- Move OnlyOffice conversion out of worker subprocesses into async event loop
  using httpx.AsyncClient with shared client and clean shutdown hook
- Add OOConversionManager with in-flight deduplication (asyncio.Future) and
  configurable concurrency limit (OO_MAX_CONCURRENT=2)
- Add 10s total timeout for office previews via asyncio.wait_for
- Return 503 when OnlyOffice is unavailable, 504 on timeout
- Remove office handling from worker dispatch(); workers now only do
  images, video, and PDFs
- Replace PNG tempfile bridge with framed binary input protocol on worker
  stdin: (json_size)(data_size)(json)(raw_data)
- Add process_image_buffer() for in-memory AVIF conversion via pyvips
- Add office_previews to WS server message, cached every 30s from OO
  availability check
- Frontend gates only office document previews on office_previews flag;
  images, video, PDFs remain unconditional
- Change default OnlyOffice port from 8080 to 8988

cista/app.py

@@ -16,7 +16,7 @@ from setproctitle import setproctitle
 from stream_zip import ZIP_AUTO, stream_zip
 from zstandard import ZstdCompressor
 
-from cista import auth, config, fileserver, preview, session, sharefs, sso, watching
+from cista import auth, config, fileserver, onlyoffice, preview, session, sharefs, sso, watching
 from cista.api import bp
 from cista.preview import shutdown_preview_workers, start_preview_workers
 from cista.sanic_logging import (
@@ -130,6 +130,7 @@ async def main_start(app):
 @app.before_server_stop
 async def main_stop(app):
     watching.stop(app)
+    await onlyoffice.close_oo_client()
     await shutdown_preview_workers()
     app.ctx.threadexec.shutdown()
     app.ctx.zipexec.shutdown(cancel_futures=True)