Docker deployment (2 images, 3 composes, Tailscale funnels) + committed security suite

Test suite (the manual audit harness, now a real test):
- backend/apps/core/tests/test_security.py: 20 transactional tests across
  guest visibility, object ownership, staged-upload/similarity privacy,
  staff role boundaries, deletion rules, encrypted credentials, throttling
  and the download allowlist. Uses temp media folders and clears cache.
  Needs a one-time GRANT on test_j621 (documented in the module + README).

Images (deploy/J621-Frontend, deploy/J621-Backend, repo root as context):
- Frontend: node build -> static nginx with SPA fallback, asset caching and
  an internal health endpoint.
- Backend: gunicorn + whitenoise (admin static collected at build), ffmpeg
  for video thumbnails, migrations applied on start, GIT_HASH build arg so
  the shell's version pill shows the commit.

Composes (distinct project names so they coexist with the dev stack):
- compose.yml (both), compose.frontend.yml, compose.backend.yml.
- A shared nginx proxy service is the only entry point (no host nginx, no
  published host ports): /api,/admin,/static,/health -> backend, everything
  else -> SPA; both upstreams resolve at request time so one config serves
  all variants.
- A Tailscale sidecar per compose shares the nginx network namespace;
  serve.default/frontend/backend.json use funnel ports 443 and 8443 only
  (10000 is the remaining allowance) with ${TS_CERT_DOMAIN} substitution.

Registry: push_frontend.sh / push_backend.sh / push_all.sh build multi-arch
images and push :latest + :<sha> to the Gitea registry, following the
existing Packs-site pattern.

Docs: deploy/README.md + .env.example, ROADMAP section 6 updated,
AGENTS.md deployment and test notes.

Verified: all three composes validate, both nginx configs pass nginx -t,
both images build, the combined stack boots against real MariaDB/Redis
(migrations applied, /health ok, whitenoise serving admin static, env=prod,
git hash baked in), the proxy serves the SPA and routes /api, and
manage.py test apps.core.tests passes 20/20.

deploy/J621-Backend

@@ -0,0 +1,40 @@
+# J621-Backend — Django + gunicorn, with migrations applied on start.
+#
+# Build context is the repository root, e.g.:
+#   docker build -f deploy/J621-Backend -t j621-backend .
+#
+# Needs MariaDB and Redis (see the compose files). ffmpeg is used for video
+# thumbnails; media/logs live under the mounted volumes.
+
+# syntax=docker/dockerfile:1
+
+FROM python:3.14-slim
+
+# Baked in by the push scripts so the shell's version pill shows the commit
+# even though the image has no .git directory.
+ARG GIT_HASH=unknown
+ENV PYTHONDONTWRITEBYTECODE=1 \
+    PYTHONUNBUFFERED=1 \
+    GIT_COMMIT_HASH=$GIT_HASH
+
+WORKDIR /app
+
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends ffmpeg \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY backend/requirements.txt ./
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY backend/ ./
+COPY deploy/backend-entrypoint.sh /usr/local/bin/j621-entrypoint
+
+RUN chmod +x /usr/local/bin/j621-entrypoint \
+    && mkdir -p /app/media /app/logs \
+    && python manage.py collectstatic --noinput
+
+EXPOSE 8000
+HEALTHCHECK --interval=30s --timeout=5s --start-period=30s \
+    CMD python -c "import urllib.request; urllib.request.urlopen('http://127.0.0.1:8000/health')" || exit 1
+
+ENTRYPOINT ["j621-entrypoint"]