Phase 3: env masking, image updates, port conflicts, resources, templates (0.3.0)

Backend:
- update_service: registry manifest digest check (Docker Hub/ghcr/lscr/private
  v2 token auth) vs local RepoDigests; in-memory cache + background loop
- port_service: parse compose ports, check /proc/net/tcp[6] + docker bindings
- template_service + bundled templates (jellyfin/vaultwarden/uptime-kuma/
  paperless-ngx/gitea) with {{VAR}} placeholders; custom templates in DB
- compose_edit set_resources (deploy.resources.limits/reservations)
- routers: images, ports, templates, editor/set-resources
- Template model; background update task wired into lifespan

Frontend:
- EnvEditor (table + raw, sensitive masking, quick-insert)
- Images page + UpdateBadge + dashboard 'updates available' banner
- PortConflictDialog pre-deploy check on Deploy
- ResourcePanel (CPU/RAM sliders) as editor Limits tab
- Templates page with per-variable instantiate form

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/templates/jellyfin.yaml

@@ -0,0 +1,14 @@
+services:
+  jellyfin:
+    image: jellyfin/jellyfin:latest
+    container_name: jellyfin
+    restart: unless-stopped
+    environment:
+      - PUID={{PUID}}
+      - PGID={{PGID}}
+      - TZ={{TZ}}
+    ports:
+      - "{{HTTP_PORT}}:8096"
+    volumes:
+      - {{CONFIG_PATH}}:/config
+      - {{MEDIA_PATH}}:/media