chore: snapshot workflow migration progress

backend/app/main.py

@@ -6,6 +6,7 @@ from fastapi.staticfiles import StaticFiles
 from pathlib import Path
 
 from app.config import settings
+from app.core.render_paths import ensure_group_writable_dir
 from app.database import engine, Base
 from app.core.websocket import manager as ws_manager
 from app.core.middleware import TenantContextMiddleware
@@ -33,7 +34,7 @@ from app.api.routers.chat import router as chat_router
 async def lifespan(app: FastAPI):
     # Create upload directories
     for subdir in ("step_files", "excel_files", "thumbnails", "renders", "blend-templates"):
-        Path(settings.upload_dir, subdir).mkdir(parents=True, exist_ok=True)
+        ensure_group_writable_dir(Path(settings.upload_dir, subdir))
     # Start WebSocket Redis subscriber
     await ws_manager.start_redis_subscriber()
     yield
@@ -59,7 +60,7 @@ app.add_middleware(TenantContextMiddleware)
 # Mount static files for thumbnails (dir created in lifespan; skip if not writable)
 thumbnails_dir = Path(settings.upload_dir) / "thumbnails"
 try:
-    thumbnails_dir.mkdir(parents=True, exist_ok=True)
+    ensure_group_writable_dir(thumbnails_dir)
     app.mount("/thumbnails", StaticFiles(directory=str(thumbnails_dir)), name="thumbnails")
 except (PermissionError, OSError):
     pass  # Running outside Docker without upload dir — thumbnails won't be served statically
@@ -67,7 +68,7 @@ except (PermissionError, OSError):
 # Mount static files for renders
 renders_dir = Path(settings.upload_dir) / "renders"
 try:
-    renders_dir.mkdir(parents=True, exist_ok=True)
+    ensure_group_writable_dir(renders_dir)
     app.mount("/renders", StaticFiles(directory=str(renders_dir)), name="renders")
 except (PermissionError, OSError):
     pass