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.

docs/docs.json

@@ -102,6 +102,7 @@
               "features/global-observability",
               "features/host-console",
               "features/multi-node",
+              "features/node-compatibility",
               "features/fleet-view",
               "features/stack-labels",
               "features/alerts-notifications",
@@ -145,9 +146,10 @@
         "pages": [
           "api-reference/overview",
           {
-            "group": "Health",
+            "group": "Health & Meta",
             "pages": [
-              "GET /api/health"
+              "GET /api/health",
+              "GET /api/meta"
             ]
           },
           {
@@ -210,7 +212,8 @@
               "GET /api/nodes/{id}",
               "PUT /api/nodes/{id}",
               "DELETE /api/nodes/{id}",
-              "POST /api/nodes/{id}/test"
+              "POST /api/nodes/{id}/test",
+              "GET /api/nodes/{id}/meta"
             ]
           },
           {