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/schemas.py

@@ -0,0 +1,58 @@
+from datetime import datetime
+from decimal import Decimal
+
+from pydantic import BaseModel, ConfigDict
+
+
+class CustomerIn(BaseModel):
+    name: str
+    email: str
+
+
+class CustomerOut(CustomerIn):
+    model_config = ConfigDict(from_attributes=True)
+    id: int
+    created_at: datetime
+
+
+class LineItemIn(BaseModel):
+    description: str
+    quantity: int = 1
+    unit_price: Decimal = Decimal("0")
+
+
+class LineItemOut(LineItemIn):
+    model_config = ConfigDict(from_attributes=True)
+    id: int
+    invoice_id: int
+
+
+class PaymentIn(BaseModel):
+    amount: Decimal
+    method: str = "cash"
+
+
+class PaymentOut(PaymentIn):
+    model_config = ConfigDict(from_attributes=True)
+    id: int
+    invoice_id: int
+    paid_at: datetime
+
+
+class InvoiceIn(BaseModel):
+    number: str
+    customer_id: int
+    due_at: datetime | None = None
+    status: str = "draft"
+
+
+class InvoiceOut(InvoiceIn):
+    model_config = ConfigDict(from_attributes=True)
+    id: int
+    issued_at: datetime
+
+
+class InvoiceDetail(InvoiceOut):
+    line_items: list[LineItemOut] = []
+    payments: list[PaymentOut] = []
+    customer: CustomerOut