add fixture-invoicing example, sample-room wrap, kind cluster support

- examples/fixture-invoicing/: FastAPI + Vue + Postgres demo (4-entity invoice fixture)
- cfg/sample/: wraps the fixture (managed.repos points at examples/)
- ctrl/kind-{up,down,status}.sh + per-room k8s render in soleprint/ctrl/k8s/
- build.py: relative repo paths, resilient rmtree, optional k8s render hook
- cfg/.gitignore: stop ignoring sample/ and standalone/ template rooms

Manifests render cleanly but kind cluster has not been run end-to-end yet.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

examples/fixture-invoicing/backend/api/line_items.py

@@ -0,0 +1,40 @@
+from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy.orm import Session
+
+from db import get_session
+from models import Invoice, LineItem
+from schemas import LineItemIn, LineItemOut
+
+router = APIRouter()
+
+
+@router.get("", response_model=list[LineItemOut])
+def list_line_items(
+    invoice_id: int | None = None, session: Session = Depends(get_session)
+):
+    query = session.query(LineItem).order_by(LineItem.id)
+    if invoice_id:
+        query = query.filter(LineItem.invoice_id == invoice_id)
+    return query.all()
+
+
+@router.post("/invoices/{invoice_id}", response_model=LineItemOut, status_code=201)
+def add_line_item(
+    invoice_id: int, payload: LineItemIn, session: Session = Depends(get_session)
+):
+    if not session.get(Invoice, invoice_id):
+        raise HTTPException(404, "invoice not found")
+    item = LineItem(invoice_id=invoice_id, **payload.model_dump())
+    session.add(item)
+    session.commit()
+    session.refresh(item)
+    return item
+
+
+@router.delete("/{line_item_id}", status_code=204)
+def delete_line_item(line_item_id: int, session: Session = Depends(get_session)):
+    item = session.get(LineItem, line_item_id)
+    if not item:
+        raise HTTPException(404, "line item not found")
+    session.delete(item)
+    session.commit()