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.

AGENTS.md

@@ -31,6 +31,15 @@ Project constraints (do not regress):
   MariaDB/Redis come from docker-compose.yml.
 - Deployment will be Docker-based (compose); do not add systemd/cron unit
   files for scheduling — use the container setup for timers/workers.
+- deploy/ is the deployment source of truth: J621-Frontend (SPA on static
+  nginx) and J621-Backend (gunicorn + whitenoise, ffmpeg, migrations on
+  start), three compose variants (both / frontend-only / backend-only) behind
+  a shared nginx proxy service, and a Tailscale sidecar per compose whose
+  serve configs only use funnel ports 443 / 8443 / 10000. Images are pushed
+  to the Gitea registry with deploy/push_*.sh (multi-arch, :latest + :sha,
+  GIT_HASH baked in for the version pill).
+- Security/permission tests live in backend/apps/core/tests and need a
+  one-time grant: GRANT ALL ON `test_j621`.* TO 'j621'@'%';
 - Same-origin and cross-origin frontends both work, and the SPA can also run
   with no backend at all: a production build asks on first start (/setup) and
   stores the answer in localStorage — a URL, "" for same origin, or "none"