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/api/routers/uploads.py

@@ -241,6 +241,7 @@ async def finalize_excel(
         included_rows,
         source_excel=str(excel_path),
         category_key=parsed_dict.get("category_key"),
+        tenant_id=getattr(user, 'tenant_id', None),
     )
 
     # 5. Seed material aliases
@@ -260,6 +261,7 @@ async def finalize_excel(
         created_by=user.id,
         source_excel=str(excel_path),
         notes=body.notes,
+        tenant_id=getattr(user, 'tenant_id', None),
     )
     db.add(order)
     await db.flush()
@@ -292,6 +294,7 @@ async def finalize_excel(
                 for c in row.get("components", [])
             ],
             cad_file_id=inherited_cad,
+            tenant_id=getattr(user, 'tenant_id', None),
         )
         db.add(item)
 
@@ -316,6 +319,7 @@ async def finalize_excel(
                 product_id=uuid.UUID(product_id),
                 output_type_id=None,
                 gewuenschte_bildnummer=row.get("gewuenschte_bildnummer"),
+                tenant_id=getattr(user, 'tenant_id', None),
             )
             db.add(line)
         else:
@@ -325,6 +329,7 @@ async def finalize_excel(
                     product_id=uuid.UUID(product_id),
                     output_type_id=type_id,
                     gewuenschte_bildnummer=row.get("gewuenschte_bildnummer"),
+                    tenant_id=getattr(user, 'tenant_id', None),
                 )
                 db.add(line)
 
@@ -410,6 +415,7 @@ async def upload_step(
         file_hash=file_hash,
         file_size=len(content),
         processing_status=ProcessingStatus.pending,
+        tenant_id=getattr(user, 'tenant_id', None),
     )
     db.add(cad_file)
     await db.commit()