perf: render pipeline optimizations — sample scaling, USD logging, persistent BVH

Task 1: Resolution-aware sample count
- Auto-scale samples for resolutions <= 1024: max(32, samples * max_dim / 2048)
- 512x512 thumbnails: 256 → 64 samples (75% GPU savings)
- Thumbnail tasks capped at 64 samples via context manager
- 2048x2048 HQ renders unchanged

Task 2: USD path preference audit + logging
- Verified USD master path is correctly preferred over GLB tessellation
- Added clear emit() messages: "Using USD master" vs "No USD master — GLB path"
- Dynamic render log label: "USD → Blender" vs "STEP → GLB → Blender"

Task 3: Persistent BVH for turntable animations
- Added scene.render.use_persistent_data = True before frame loop
- BVH acceleration structure cached between frames (not rebuilt per frame)
- Applies to both camera orbit and object rotation modes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/domains/pipeline/tasks/render_thumbnail.py

@@ -14,6 +14,40 @@ from app.core.pipeline_logger import PipelineLogger
 
 logger = logging.getLogger(__name__)
 
+# Maximum samples for thumbnail renders (512x512).
+# Full-resolution renders use 256+ samples; thumbnails don't need more than 64.
+_THUMBNAIL_SAMPLE_CAP = 64
+
+
+@contextmanager
+def _capped_thumbnail_samples():
+    """Temporarily cap render samples for thumbnail renders.
+
+    Thumbnails are 512x512 — using 256 Cycles samples is wasteful.
+    This patches _get_all_settings in step_processor to cap samples
+    at _THUMBNAIL_SAMPLE_CAP for the duration of the thumbnail render.
+    """
+    import app.services.step_processor as _sp
+    _original = _sp._get_all_settings
+
+    def _patched() -> dict[str, str]:
+        settings = _original()
+        for key in ("blender_cycles_samples", "blender_eevee_samples"):
+            try:
+                val = int(settings.get(key, "256"))
+                if val > _THUMBNAIL_SAMPLE_CAP:
+                    logger.info("Capping thumbnail %s: %d -> %d", key, val, _THUMBNAIL_SAMPLE_CAP)
+                    settings[key] = str(_THUMBNAIL_SAMPLE_CAP)
+            except (ValueError, TypeError):
+                pass
+        return settings
+
+    _sp._get_all_settings = _patched
+    try:
+        yield
+    finally:
+        _sp._get_all_settings = _original
+
 
 @contextmanager
 def _pipeline_session(tenant_id: str | None = None):
@@ -67,11 +101,12 @@ def render_step_thumbnail(self, cad_file_id: str):
     except Exception:
         logger.warning(f"step_file_hash computation failed for {cad_file_id} (non-fatal)")
 
-    # ── Render thumbnail ──────────────────────────────────────────────────
+    # ── Render thumbnail (with capped samples for 512x512) ──────────────
     try:
         from app.services.step_processor import regenerate_cad_thumbnail
         pl.info("render_step_thumbnail", "Calling regenerate_cad_thumbnail")
-        success = regenerate_cad_thumbnail(cad_file_id, part_colors={})
+        with _capped_thumbnail_samples():
+            success = regenerate_cad_thumbnail(cad_file_id, part_colors={})
         if not success:
             raise RuntimeError("regenerate_cad_thumbnail returned False")
     except Exception as exc:
@@ -166,7 +201,8 @@ def regenerate_thumbnail(self, cad_file_id: str, part_colors: dict):
 
     try:
         from app.services.step_processor import regenerate_cad_thumbnail
-        success = regenerate_cad_thumbnail(cad_file_id, part_colors)
+        with _capped_thumbnail_samples():
+            success = regenerate_cad_thumbnail(cad_file_id, part_colors)
         if not success:
             raise RuntimeError("regenerate_cad_thumbnail returned False")
     except Exception as exc: