feat(updates): NetBird container image update management

- New image_service.py: Docker Hub digest check (no pull), local digest/ID
  comparison, pull_all_images, per-customer container image status, and
  update_customer_containers (docker compose up -d, data-safe)
- Monitoring endpoints: GET /images/check (hub vs local + per-customer
  needs_update), POST /images/pull (background), POST /customers/update-all
- Deployment endpoint: POST /{id}/update-images (single-customer update)
- Monitoring page: "NetBird Container Updates" card with Check / Pull / Update
  All buttons; image status table and per-customer update table with inline
  update buttons
- i18n: added keys in en.json and de.json

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

static/lang/en.json

@@ -280,7 +280,26 @@
     "thDashboard": "Dashboard",
     "thRelayPort": "Relay Port",
     "thContainers": "Containers",
-    "noCustomers": "No customers."
+    "noCustomers": "No customers.",
+    "imageUpdates": "NetBird Container Updates",
+    "checkUpdates": "Check for Updates",
+    "pullImages": "Pull Latest Images",
+    "updateAll": "Update All",
+    "clickCheckUpdates": "Click \"Check for Updates\" to compare local images with Docker Hub.",
+    "updateAvailable": "Update available",
+    "upToDate": "Up to date",
+    "needsUpdate": "Needs update",
+    "updateCustomer": "Update this customer",
+    "imageStatusTitle": "Image Status (vs. Docker Hub)",
+    "customerImageTitle": "Customer Container Status",
+    "thImage": "Image",
+    "thLocalDigest": "Local Digest",
+    "thHubDigest": "Hub Digest",
+    "confirmPull": "Pull the latest NetBird images from Docker Hub? This may take a few minutes.",
+    "pullStarted": "Image pull started in background. Re-checking in 5 seconds…",
+    "confirmUpdateAll": "Recreate containers for all customers that have outdated images? Running services will briefly restart.",
+    "updateAllStarted": "Update started in background.",
+    "updateDone": "Customer containers updated."
   },
   "userModal": {
     "title": "New User",