refactor(A2): replace blender-renderer HTTP service with render-worker Celery container

- Create render-worker/ with Dockerfile (Ubuntu + cadquery + Blender via host mount)
- Add render-worker/check_version.py: verifies Blender >= 5.0.1 at startup, Exit 1 on failure
- Add render-worker/scripts/: blender_render.py, still_render.py, turntable_render.py
- Create backend/app/services/render_blender.py: direct subprocess rendering
  - convert_step_to_stl() and export_per_part_stls() using cadquery
  - render_still(): STEP → STL → PNG via Blender subprocess
  - is_blender_available(): detects BLENDER_BIN env for render-worker context
- Create backend/app/domains/rendering/tasks.py: render_still_task + render_turntable_task
- Update step_processor.py: use subprocess path when BLENDER_BIN env is set (render-worker)
- Update step_tasks.py: generate_stl_cache uses direct cadquery instead of HTTP
- Remove blender-renderer and threejs-renderer from docker-compose.yml
- Replace worker-thumbnail with render-worker (Ubuntu + cadquery + Blender mount)
- Remove Docker SDK from backend Dockerfile (was only for flamenco scaling)
- Update .env.example: BLENDER_VERSION=5.0.1 documented
- Update celery_app.py: include domains.rendering.tasks in autodiscover

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

render-worker/check_version.py

@@ -0,0 +1,68 @@
+"""Startup check: verify Blender >= 5.0.1 is available.
+
+Run before starting the Celery worker. Exits with code 1 if Blender is
+missing or below the minimum required version.
+"""
+import os
+import re
+import subprocess
+import sys
+from pathlib import Path
+
+MIN_VERSION = (5, 0, 1)
+MIN_VERSION_STR = ".".join(str(v) for v in MIN_VERSION)
+
+
+def find_blender() -> str:
+    import shutil
+    env_bin = os.environ.get("BLENDER_BIN", "")
+    if env_bin and Path(env_bin).exists():
+        return env_bin
+    found = shutil.which("blender")
+    return found or "blender"
+
+
+def check_version():
+    blender_bin = find_blender()
+
+    if not Path(blender_bin).exists():
+        print(f"ERROR: Blender not found at {blender_bin}", file=sys.stderr)
+        print(
+            "Mount Blender >= 5.0.1 from the host via:\n"
+            "  volumes:\n"
+            "    - /opt/blender:/opt/blender:ro",
+            file=sys.stderr,
+        )
+        sys.exit(1)
+
+    try:
+        result = subprocess.run(
+            [blender_bin, "--version"],
+            capture_output=True, text=True, timeout=15
+        )
+        output = result.stdout or result.stderr or ""
+    except Exception as exc:
+        print(f"ERROR: Could not run Blender: {exc}", file=sys.stderr)
+        sys.exit(1)
+
+    match = re.search(r"Blender\s+(\d+)\.(\d+)\.(\d+)", output)
+    if not match:
+        print(f"ERROR: Could not parse Blender version from output:\n{output[:200]}", file=sys.stderr)
+        sys.exit(1)
+
+    version = tuple(int(x) for x in match.groups())
+    version_str = ".".join(str(v) for v in version)
+
+    if version < MIN_VERSION:
+        print(
+            f"ERROR: Blender {version_str} < required {MIN_VERSION_STR}.\n"
+            f"Update your host Blender installation.",
+            file=sys.stderr,
+        )
+        sys.exit(1)
+
+    print(f"Blender {version_str} OK (>= {MIN_VERSION_STR})")
+
+
+if __name__ == "__main__":
+    check_version()