Containerize the API with a persistent volume

The API deployable had no packaging of its own: its runtime deps lived
only in the integration's dev group, so there was no way to deploy the
service without dragging the test venv along. Package the API as a
container image (Dockerfile, multi-stage slim build, non-root) with the
deployable's own manifest api/requirements.txt, and run it via
deploy/compose.yaml with the SQLite file on the named volume
nestquest-db so recreating the container keeps household data.
Secrets are supplied by the environment (ignored deploy/.env, example
with placeholder names only). TLS/reverse proxy, Authentik wiring and
network separation are later Feature 17 tasks; the port is bound to
localhost until they land.

.gitignore

@@ -5,3 +5,12 @@ __pycache__/
 .pytest_cache/
 *.metadata.json
 frontend/node_modules/
+
+# Feature 17 (API deployment): local secret config and runtime data
+# are never committed — compose reads deploy/.env; SQLite/WAL sidecar
+# files are container/volume artifacts.
+.env
+**/.env
+*.db
+*.db-wal
+*.db-shm