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/orders.py

@@ -345,6 +345,7 @@ async def create_order(
         created_by=user.id,
         source_excel=body.source_excel,
         notes=body.notes,
+        tenant_id=getattr(user, 'tenant_id', None),
     )
     db.add(order)
     await db.flush()
@@ -364,6 +365,7 @@ async def create_order(
             lagertyp=item_data.lagertyp,
             medias_rendering=item_data.medias_rendering,
             components=[c.model_dump() for c in item_data.components],
+            tenant_id=getattr(user, 'tenant_id', None),
         )
         db.add(item)
 
@@ -381,6 +383,7 @@ async def create_order(
             render_position_id=line_data.render_position_id,
             gewuenschte_bildnummer=line_data.gewuenschte_bildnummer,
             notes=line_data.notes,
+            tenant_id=getattr(user, 'tenant_id', None),
         )
         db.add(line)
 
@@ -565,6 +568,7 @@ async def split_missing_step(
         created_by=order.created_by,
         source_excel=order.source_excel,
         notes=f"Split from {order.order_number} — awaiting STEP files",
+        tenant_id=order.tenant_id,
     )
     db.add(new_order)
     await db.flush()
@@ -665,6 +669,7 @@ async def generate_lines_from_items(
                 product_id=product.id,
                 output_type_id=type_id,
                 gewuenschte_bildnummer=item.gewuenschte_bildnummer,
+                tenant_id=getattr(user, 'tenant_id', None),
             )
             db.add(line)
             existing_pairs.add(pair)
@@ -807,6 +812,7 @@ async def add_order_line(
         render_position_id=body.render_position_id,
         gewuenschte_bildnummer=body.gewuenschte_bildnummer,
         notes=body.notes,
+        tenant_id=getattr(user, 'tenant_id', None),
     )
     db.add(line)
     try: