Containerize the API with a persistent volume (#183)

Dockerfile

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+# Container image for the NestQuest API service (Feature 17).
+#
+# Build context is the REPOSITORY ROOT (compose sets it), because the
+# API loads the bundled domain layer from
+# custom_components/nestquest/core/ by file location
+# (api/nestquest_core.py): the image must carry both api/ and the
+# bundled core at their relative positions.  Everything else in the
+# repository is excluded via .dockerignore.
+#
+# Multi-stage so no pip/build toolchain survives into the runtime:
+# the builder installs the deployable's own manifest
+# (api/requirements.txt) into a staging prefix and the runtime copies
+# only the installed packages.  The service runs as a non-root user;
+# the SQLite database lives on the mounted volume at
+# /var/lib/nestquest (compose supplies NESTQUEST_DB_PATH pointing
+# there — configuration is environment-supplied, never baked in).
+FROM python:3.12-slim AS builder
+
+COPY api/requirements.txt /tmp/requirements.txt
+RUN pip install --no-cache-dir --prefix=/install -r /tmp/requirements.txt
+
+FROM python:3.12-slim
+
+# The staged dependency install from the builder (no pip, no wheels).
+COPY --from=builder /install /usr/local
+
+# /app is the repository-root equivalent: api/ resolves imports as
+# ``api.*`` and nestquest_core.py finds the bundled core one level up.
+WORKDIR /app
+COPY api/ ./api/
+COPY custom_components/nestquest/core/ ./custom_components/nestquest/core/
+
+# Non-root runtime user; owns the database volume mount point.
+RUN useradd --system --create-home --home-dir /home/nestquest nestquest \
+    && mkdir -p /var/lib/nestquest \
+    && chown nestquest:nestquest /var/lib/nestquest
+USER nestquest
+
+ENV PYTHONUNBUFFERED=1 PYTHONDONTWRITEBYTECODE=1
+
+# The database path and the service token / OIDC settings are supplied
+# by the environment (compose env_file), never committed into the image.
+ENV NESTQUEST_DB_PATH=/var/lib/nestquest/nestquest.db
+VOLUME /var/lib/nestquest
+
+EXPOSE 8080
+CMD ["uvicorn", "--factory", "api.app:create_app", "--host", "0.0.0.0", "--port", "8080"]