fix: render pipeline + multi-tenancy bugs (B-Fix-1 through B-Fix-9)

- Remove worker-thumbnail (no Blender, was competing on thumbnail_rendering)
- Move render_order_line_task to thumbnail_rendering queue (render-worker)
- Restore template_service.py real implementation (fix circular import shim)
- Thread tenant_id through STEP upload, Excel import, product create
- Make system tables (output_types, materials, etc.) tenant_id nullable
- Fix tenants frontend 307-redirect: use trailing slash /tenants/
- Remove Flamenco + Three.js from Admin UI (unsupported)
- Set all output_types render_backend to celery (was flamenco)

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

backend/app/domains/products/service.py

@@ -69,7 +69,7 @@ async def lookup_product(
 
 
 async def lookup_or_create_product(
-    db: AsyncSession, pim_id: str | None, fields: dict
+    db: AsyncSession, pim_id: str | None, fields: dict, tenant_id=None
 ) -> tuple[Product, bool]:
     """Look up by produkt_baureihe (primary), then pim_id (fallback). Create if not found.
 
@@ -120,6 +120,7 @@ async def lookup_or_create_product(
         components=fields.get("components", []),
         cad_part_materials=fields.get("cad_part_materials", []),
         source_excel=fields.get("source_excel"),
+        tenant_id=tenant_id,
     )
     db.add(product)
     await db.flush()

backend/app/domains/rendering/dispatch_service.py

@@ -110,9 +110,7 @@ def dispatch_render_with_workflow(order_line_id: str) -> dict:
 
 
 def _legacy_dispatch(order_line_id: str) -> dict:
-    """Delegate to the original render_dispatcher logic (kept for backward compat)."""
-    # Import the original full implementation (not the shim) to avoid circular imports.
-    # The original logic lives inline in the orders router / step_tasks path;
-    # here we re-use the existing flamenco/celery routing code.
-    from app.services.render_dispatcher import dispatch_render  # noqa: F401 — shim re-export
-    return dispatch_render(order_line_id)
+    """Queue render_order_line_task (the working Celery render implementation)."""
+    from app.tasks.step_tasks import render_order_line_task
+    render_order_line_task.delay(order_line_id)
+    return {"backend": "celery", "queued": True}

backend/app/domains/rendering/service.py

@@ -1,6 +1,7 @@
 """Rendering services — template resolution, dispatch, and Blender utilities."""
 
-# Re-export from original service files for backward compatibility.
+# Re-export from canonical service files for backward compatibility.
+# template_service contains the actual sync implementations (Celery-safe).
 from app.services.template_service import resolve_template, get_material_library_path
 from app.services.render_dispatcher import dispatch_render
 from app.services.render_blender import find_blender, is_blender_available