Feature 17 review remediation: commit firewall unit, pin image and deps

F17-001: commit the API box's nestquest-api-firewall.service verbatim
as deploy/systemd/nestquest-api-firewall.service (DOCKER-USER RETURN
for 10.60.1.33 and 10.60.1.14, then DROP tcp/8080) and make the
runbook install it from the repository before exposing the API.

F17-002: pin the python:3.12-slim base image by digest in both build
stages, pin api/requirements.txt to the exact versions installed in
the deployed image (direct deps plus transitive closure), and pin the
litestream image by digest in deploy/compose.yaml.

Task: e4a7b18b-5dad-4387-90ac-33588c280824

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -14,12 +14,16 @@
 # 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
+#
+# Both stages use the same base image pinned by digest, and
+# api/requirements.txt pins exact versions, so a rebuild reproduces
+# the deployed image instead of floating to whatever is newest.
+FROM python:3.12-slim@sha256:2f17fc044b579bab302c2e8054d3a686e2cb9a83de48e70534b94cd8ebbe06a9 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
+FROM python:3.12-slim@sha256:2f17fc044b579bab302c2e8054d3a686e2cb9a83de48e70534b94cd8ebbe06a9
 
 # The staged dependency install from the builder (no pip, no wheels).
 COPY --from=builder /install /usr/local