fix: ensure temp files use shared volume with correct permissions

- Modified render_accessible_video.py to explicitly pass TMPDIR to
  tempfile.TemporaryDirectory() so files are created in shared volume
- Updated docker-compose.yml to run containers as root initially,
  chown /shared-tmp to app:app, then switch to app user for celery
- This ensures both worker containers can access the same temp files

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/tasks/render_accessible_video.py

@@ -91,7 +91,9 @@ async def _async_render_accessible_video(job_id: str, language: str):
             message=f"Rendering accessible video for {language.upper()}"
         )
 
-        with tempfile.TemporaryDirectory() as temp_dir:
+        # Use TMPDIR env var if set (for shared volume between workers)
+        temp_base = os.environ.get('TMPDIR', None)
+        with tempfile.TemporaryDirectory(dir=temp_base) as temp_dir:
             # 1. Download source video from GCS
             source_video_gcs = job_doc["source"]["gcs_uri"]
             source_blob_path = source_video_gcs.replace(f"gs://{settings.gcs_bucket}/", "")

docker-compose.yml

@@ -151,7 +151,10 @@ services:
       target: worker
     container_name: accessible-video-worker
     restart: unless-stopped
-    command: ["celery", "-A", "celery_worker", "worker", "-Q", "default,ingest,notify,render", "--loglevel=info", "--concurrency=4"]
+    user: root
+    command: >
+      sh -c "chown -R app:app /shared-tmp &&
+             su app -c 'celery -A celery_worker worker -Q default,ingest,notify,render --loglevel=info --concurrency=4'"
     depends_on:
       mongodb:
         condition: service_healthy
@@ -229,7 +232,10 @@ services:
       target: worker
     container_name: accessible-video-ffmpeg-worker
     restart: unless-stopped
-    command: ["celery", "-A", "celery_worker", "worker", "-Q", "ffmpeg", "--loglevel=info", "--concurrency=1"]
+    user: root
+    command: >
+      sh -c "chown -R app:app /shared-tmp 2>/dev/null || true &&
+             su app -c 'celery -A celery_worker worker -Q ffmpeg --loglevel=info --concurrency=1'"
     depends_on:
       mongodb:
         condition: service_healthy