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/db.py

@@ -0,0 +1,29 @@
+import os
+
+from sqlalchemy import create_engine
+from sqlalchemy.orm import DeclarativeBase, sessionmaker
+
+
+def _db_url() -> str:
+    host = os.getenv("DB_HOST", "localhost")
+    port = os.getenv("DB_PORT", "5432")
+    name = os.getenv("DB_NAME", "fixture")
+    user = os.getenv("DB_USER", "postgres")
+    password = os.getenv("DB_PASSWORD", "fixture")
+    return f"postgresql+psycopg2://{user}:{password}@{host}:{port}/{name}"
+
+
+engine = create_engine(_db_url(), echo=False, future=True)
+SessionLocal = sessionmaker(bind=engine, autoflush=False, autocommit=False, future=True)
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+def get_session():
+    session = SessionLocal()
+    try:
+        yield session
+    finally:
+        session.close()