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/tasks/step_tasks.py

@@ -239,42 +239,12 @@ def regenerate_thumbnail(self, cad_file_id: str, part_colors: dict):
 
 @celery_app.task(name="app.tasks.step_tasks.dispatch_order_line_render", queue="step_processing")
 def dispatch_order_line_render(order_line_id: str):
-    """Thin wrapper that calls render_dispatcher.dispatch_render()."""
+    """Route an order-line render to render_order_line_task."""
     logger.info(f"Dispatching render for order line: {order_line_id}")
-    try:
-        from app.services.render_dispatcher import dispatch_render
-        result = dispatch_render(order_line_id)
-        logger.info(f"Dispatch result for {order_line_id}: {result}")
-        return result
-    except Exception as exc:
-        logger.error(f"dispatch_order_line_render failed for {order_line_id}: {exc}")
-        # Mark line as failed so it doesn't stay stuck in "processing"
-        try:
-            from sqlalchemy import create_engine, update as sql_update
-            from sqlalchemy.orm import Session
-            from app.config import settings as app_settings
-            from app.models.order_line import OrderLine
-            from datetime import datetime
-            sync_url = app_settings.database_url.replace("+asyncpg", "")
-            eng = create_engine(sync_url)
-            with Session(eng) as s:
-                s.execute(
-                    sql_update(OrderLine)
-                    .where(OrderLine.id == order_line_id)
-                    .values(
-                        render_status="failed",
-                        render_completed_at=datetime.utcnow(),
-                        render_log={"error": f"Dispatch failed: {str(exc)[:500]}"},
-                    )
-                )
-                s.commit()
-            eng.dispose()
-        except Exception:
-            logger.exception(f"Failed to mark {order_line_id} as failed after dispatch error")
-        raise
+    render_order_line_task.delay(order_line_id)
 
 
-@celery_app.task(bind=True, name="app.tasks.step_tasks.render_order_line_task", queue="step_processing", max_retries=3)
+@celery_app.task(bind=True, name="app.tasks.step_tasks.render_order_line_task", queue="thumbnail_rendering", max_retries=3)
 def render_order_line_task(self, order_line_id: str):
     """Render a specific output type for an order line.