merge upstream: system status, health monitoring, connectivity caching

Merges xmrcheckout/xmrcheckout main (7 commits since 655d037):
- Add Monero system status and reconciler heartbeat
- Surface Monero health status in UI (dashboard + invoice pages)
- Harden reconciler status heartbeat startup
- Avoid false Monero outage states in UI
- Cache Monero connectivity in system status
- Next.js security patch (15.5.9 → 15.5.14)
- Bump Monero Docker defaults to 0.18.4.6

Resolved conflicts in reconciler.py (kept our advisory lock + upstream
heartbeat), routes.py (kept both imports), README.md (kept both sections).

ui/package.json

@@ -11,7 +11,7 @@
   "dependencies": {
     "doctrine": "^3.0.0",
     "esutils": "^2.0.3",
-    "next": "15.5.9",
+    "next": "15.5.14",
     "qrcode": "1.5.4",
     "react": "19.2.3",
     "@marsidev/react-turnstile": "^1.4.2",
@@ -26,7 +26,7 @@
     "@types/react-dom": "18.3.0",
     "autoprefixer": "^10.4.23",
     "eslint": "9.9.0",
-    "eslint-config-next": "15.5.9",
+    "eslint-config-next": "15.5.14",
     "postcss": "^8.5.6",
     "tailwindcss": "^3.4.14",
     "typescript": "5.5.4"