feat(F1): wire MinIO STL cache into render_still + render_turntable_to_file

Previously the cache_service was only used in the generate_stl_cache Celery task.
All render paths (render_still, render_turntable_to_file, render_turntable_task)
only checked for a local file and converted from scratch if missing.

Changes:
- render_blender.py: add _stl_from_cache_or_convert() helper that checks MinIO
  cache before falling back to local STEP→STL conversion. Wire into render_still()
  and render_turntable_to_file() (both STL conversion blocks).
- domains/rendering/tasks.py: wire MinIO cache check into render_turntable_task()
  inline before convert_step_to_stl(). All errors are non-fatal (falls back to
  fresh conversion).

Now a STEP file converted on one worker is available to all workers via MinIO,
avoiding redundant cadquery conversions on re-renders.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/domains/rendering/tasks.py

@@ -155,10 +155,21 @@ def render_turntable_task(
     scripts_dir = Path(os.environ.get("RENDER_SCRIPTS_DIR", "/render-scripts"))
     turntable_script = scripts_dir / "turntable_render.py"
 
-    # STL conversion
+    # STL conversion — try MinIO cache first, then convert locally
     stl_path = step.parent / f"{step.stem}_{stl_quality}.stl"
     if not stl_path.exists() or stl_path.stat().st_size == 0:
-        convert_step_to_stl(step, stl_path, stl_quality)
+        try:
+            from app.domains.products.cache_service import compute_step_hash, check_stl_cache
+            step_hash = compute_step_hash(str(step))
+            cached = check_stl_cache(step_hash, stl_quality)
+            if cached:
+                stl_path.write_bytes(cached)
+                logger.info("STL restored from MinIO cache: %s", stl_path.name)
+            else:
+                convert_step_to_stl(step, stl_path, stl_quality)
+        except Exception as exc:
+            logger.warning("MinIO cache check failed (non-fatal): %s — falling back to conversion", exc)
+            convert_step_to_stl(step, stl_path, stl_quality)
     parts_dir = step.parent / f"{step.stem}_{stl_quality}_parts"
     if not (parts_dir / "manifest.json").exists():
         try: