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/api.py

@@ -55,6 +55,8 @@ async def watch(req, ws):
             "privileged": req.ctx.user.privileged,
         }
 
+    from cista import onlyoffice
+
     await ws.send(
         msgspec.json.encode(
             {
@@ -63,6 +65,7 @@ async def watch(req, ws):
                     "version": __version__,
                     "public": config.config.public,
                     "paskia": sso.paskia_enabled(),
+                    "office_previews": await onlyoffice.is_available_cached(),
                 },
                 "user": user_info,
             }