Keep secrets and runtime data out of the Docker images

The build context is the repository root and there was no .dockerignore, so
'COPY backend/ ./' swept backend/venv (327 MB), backend/media (the actual
library, 224 MB), backend/logs, backend/staticfiles and backend/.env
(SECRET_KEY plus the database password) into the backend image: 1.43 GB per
architecture, including secrets headed for the registry. The frontend build
stage also copied the host's node_modules over the fresh install.

- Root .dockerignore excludes .git, virtualenvs, __pycache__, db.sqlite3,
  logs/staticfiles, .env files, media/, node_modules, dist and the deploy
  runtime state (data/, tailscale-state/).
- The backend Dockerfile now asserts the context is clean (.env, venv,
  media/library, db.sqlite3 all absent) before collectstatic, so a missing
  ignore file fails the build instead of leaking.
- Rebuilt: backend 1.43 GB -> 876 MB ('COPY backend/' is now 268 kB),
  frontend stays at 65 MB. Verified by booting the compose stack with the
  new image: migrations applied, /health ok, no .env or venv inside, and
  /app/media is the mounted (empty) volume; the scheduler runs too.
- Removed the stale local images that still contained the library and the
  dev .env.

deploy/J621-Backend

@@ -30,6 +30,14 @@ COPY backend/ ./
 COPY deploy/backend-entrypoint.sh /usr/local/bin/j621-entrypoint
 COPY deploy/scheduler-entrypoint.sh /usr/local/bin/j621-scheduler
 
+# Guard: fail the build if the context leaked secrets or runtime data
+# (.dockerignore excludes them — see the repository root).
+RUN test ! -e /app/.env \
+    && test ! -d /app/venv \
+    && test ! -d /app/media/library \
+    && test ! -e /app/db.sqlite3 \
+    && echo "build context clean"
+
 RUN chmod +x /usr/local/bin/j621-entrypoint /usr/local/bin/j621-scheduler \
     && mkdir -p /app/media /app/logs \
     && python manage.py collectstatic --noinput