feat(nodes): add capability-based node compatibility negotiation (#350)

* feat(nodes): add capability-based node compatibility negotiation

Each Sencho instance now exposes /api/meta with its version and supported
capabilities. When the user switches nodes, the frontend fetches this
metadata and disables features the remote node doesn't support via a
CapabilityGate overlay. Version is shown in the node switcher dropdown
and connection test results.

- Backend: CapabilityRegistry with static capability list and fetchRemoteMeta helper
- Backend: /api/meta (public) and /api/nodes/:id/meta (auth) endpoints
- Frontend: NodeContext enhanced with per-node meta caching (5min TTL)
- Frontend: CapabilityGate component with typed Capability union
- Frontend: 13 features wrapped with capability gates
- Docs: node-compatibility.mdx + OpenAPI spec updates

* fix(nodes): revert to require() for package.json version reading

The static import fails in the Docker multi-stage build because the
root package.json is not copied into the backend-builder stage. The
require() call resolves at runtime when the file is available.

frontend/src/components/settings/NotificationRoutingSection.tsx

@@ -29,6 +29,7 @@ import {
 import { toast } from '@/components/ui/toast-store';
 import { apiFetch } from '@/lib/api';
 import { AdmiralGate } from '@/components/AdmiralGate';
+import { CapabilityGate } from '@/components/CapabilityGate';
 import { TierBadge } from '@/components/TierBadge';
 import { Plus, Trash2, Pencil, RefreshCw, Zap, X, GitBranch } from 'lucide-react';
 
@@ -230,6 +231,7 @@ export function NotificationRoutingSection() {
 
     return (
         <AdmiralGate featureName="Notification Routing">
+          <CapabilityGate capability="notification-routing" featureName="Notification Routing">
             <div className="space-y-6">
                 <div className="flex items-start justify-between pr-8">
                     <div>
@@ -452,6 +454,7 @@ export function NotificationRoutingSection() {
                     </div>
                 ))}
             </div>
+          </CapabilityGate>
         </AdmiralGate>
     );
 }