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

@@ -155,9 +155,20 @@ 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:
+        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():

backend/app/services/render_blender.py

@@ -17,6 +17,26 @@ logger = logging.getLogger(__name__)
 MIN_BLENDER_VERSION = (5, 0, 1)
 
 
+def _stl_from_cache_or_convert(step_path: Path, stl_path: Path, quality: str) -> None:
+    """Try MinIO cache first, then fall back to local STEP→STL conversion."""
+    # MinIO cache check (non-fatal — cache miss just means we convert normally)
+    try:
+        from app.domains.products.cache_service import compute_step_hash, check_stl_cache
+        step_hash = compute_step_hash(str(step_path))
+        cached_bytes = check_stl_cache(step_hash, quality)
+        if cached_bytes:
+            stl_path.write_bytes(cached_bytes)
+            logger.info("STL restored from MinIO cache: %s (%d KB)", stl_path.name, len(cached_bytes) // 1024)
+            return
+    except Exception as exc:
+        logger.warning("MinIO cache check failed (non-fatal): %s", exc)
+
+    # Local conversion
+    from app.services.step_processor import convert_step_to_stl
+    logger.info("STL cache miss — converting: %s", step_path.name)
+    convert_step_to_stl(step_path, stl_path, quality)
+
+
 def find_blender() -> str:
     """Locate the Blender binary via $BLENDER_BIN or PATH."""
     env_bin = os.environ.get("BLENDER_BIN", "")
@@ -210,10 +230,9 @@ def render_still(
 
     t_stl = time.monotonic()
     if not stl_path.exists() or stl_path.stat().st_size == 0:
-        logger.info("STL cache miss — converting: %s", step_path.name)
-        convert_step_to_stl(step_path, stl_path, stl_quality)
+        _stl_from_cache_or_convert(step_path, stl_path, stl_quality)
     else:
-        logger.info("STL cache hit: %s (%d KB)", stl_path.name, stl_path.stat().st_size // 1024)
+        logger.info("STL local hit: %s (%d KB)", stl_path.name, stl_path.stat().st_size // 1024)
     stl_size_bytes = stl_path.stat().st_size if stl_path.exists() else 0
 
     if not (parts_dir / "manifest.json").exists():
@@ -394,10 +413,9 @@ def render_turntable_to_file(
 
     t_stl = time.monotonic()
     if not stl_path.exists() or stl_path.stat().st_size == 0:
-        logger.info("STL cache miss — converting: %s", step_path.name)
-        convert_step_to_stl(step_path, stl_path, stl_quality)
+        _stl_from_cache_or_convert(step_path, stl_path, stl_quality)
     else:
-        logger.info("STL cache hit: %s (%d KB)", stl_path.name, stl_path.stat().st_size // 1024)
+        logger.info("STL local hit: %s (%d KB)", stl_path.name, stl_path.stat().st_size // 1024)
     stl_size_bytes = stl_path.stat().st_size if stl_path.exists() else 0
 
     if not (parts_dir / "manifest.json").exists():