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/WebhooksSection.tsx

@@ -9,6 +9,7 @@ import { Select, SelectContent, SelectItem, SelectTrigger, SelectValue } from '@
 import { toast } from '@/components/ui/toast-store';
 import { apiFetch } from '@/lib/api';
 import { ProGate } from '@/components/ProGate';
+import { CapabilityGate } from '@/components/CapabilityGate';
 import { TierBadge } from '@/components/TierBadge';
 import {
     RefreshCw, CheckCircle, XCircle, Webhook, Copy, Trash2,
@@ -141,10 +142,12 @@ export function WebhooksSection({ isPro }: { isPro: boolean }) {
                     <p className="text-sm text-muted-foreground">Trigger stack actions from CI/CD pipelines via HTTP.</p>
                 </div>
                 <ProGate featureName="Webhooks">
+                  <CapabilityGate capability="webhooks" featureName="Webhooks">
                     <div className="space-y-3">
                         <div className="h-16 rounded-lg border bg-card" />
                         <div className="h-16 rounded-lg border bg-card" />
                     </div>
+                  </CapabilityGate>
                 </ProGate>
             </div>
         );