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()