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/main.py

@@ -0,0 +1,52 @@
+import logging
+import os
+
+from fastapi import FastAPI
+from fastapi.middleware.cors import CORSMiddleware
+
+from api import customers, invoices, line_items, payments
+from db import Base, engine
+from seed import seed_if_empty
+
+logging.basicConfig(level=logging.INFO, format="%(levelname)s %(name)s: %(message)s")
+log = logging.getLogger("fixture")
+
+app = FastAPI(
+    title="Fixture Invoicing",
+    description="SOLEPRINT FIXTURE APP — not a real invoicing product",
+    version="0.1.0",
+)
+
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=["*"],
+    allow_methods=["*"],
+    allow_headers=["*"],
+)
+
+
+@app.on_event("startup")
+def startup():
+    Base.metadata.create_all(bind=engine)
+    if os.getenv("SEED_ON_START", "false").lower() == "true":
+        seed_if_empty()
+
+
+@app.get("/")
+def root():
+    return {
+        "app": "fixture-invoicing",
+        "warning": "THIS IS A SOLEPRINT FIXTURE — NOT A REAL PRODUCT",
+        "docs": "/docs",
+    }
+
+
+@app.get("/health")
+def health():
+    return {"status": "ok"}
+
+
+app.include_router(customers.router, prefix="/api/customers", tags=["customers"])
+app.include_router(invoices.router, prefix="/api/invoices", tags=["invoices"])
+app.include_router(line_items.router, prefix="/api/line-items", tags=["line-items"])
+app.include_router(payments.router, prefix="/api/payments", tags=["payments"])