feat(meta): gate deferred Fleet tabs behind SENCHO_EXPERIMENTAL flag (#886)

Hide the Traffic / Routing, Deployments, Federation and Secrets Fleet
tabs by default. They re-appear when the operator opts in by setting
SENCHO_EXPERIMENTAL=true. Backend routes and database tables are
unchanged; this is a UI discovery gate only.

The /api/meta endpoint now returns experimental as a boolean. A new
useExperimental hook reads it once per page load and feeds the four
tab triggers and tab content panels in FleetView.

backend/src/__tests__/health.test.ts

@@ -39,3 +39,54 @@ describe('GET /api/health', () => {
     expect(res.status).not.toBe(403);
   });
 });
+
+describe('GET /api/meta experimental flag', () => {
+  it('reports experimental=false by default', async () => {
+    const prev = process.env.SENCHO_EXPERIMENTAL;
+    delete process.env.SENCHO_EXPERIMENTAL;
+    try {
+      const res = await request(app).get('/api/meta');
+      expect(res.status).toBe(200);
+      expect(res.body.experimental).toBe(false);
+    } finally {
+      if (prev !== undefined) process.env.SENCHO_EXPERIMENTAL = prev;
+    }
+  });
+
+  it('reports experimental=true when SENCHO_EXPERIMENTAL=true', async () => {
+    const prev = process.env.SENCHO_EXPERIMENTAL;
+    process.env.SENCHO_EXPERIMENTAL = 'true';
+    try {
+      const res = await request(app).get('/api/meta');
+      expect(res.status).toBe(200);
+      expect(res.body.experimental).toBe(true);
+    } finally {
+      if (prev === undefined) delete process.env.SENCHO_EXPERIMENTAL;
+      else process.env.SENCHO_EXPERIMENTAL = prev;
+    }
+  });
+
+  it('treats any non-"true" value as false', async () => {
+    const prev = process.env.SENCHO_EXPERIMENTAL;
+    process.env.SENCHO_EXPERIMENTAL = '1';
+    try {
+      const res = await request(app).get('/api/meta');
+      expect(res.body.experimental).toBe(false);
+    } finally {
+      if (prev === undefined) delete process.env.SENCHO_EXPERIMENTAL;
+      else process.env.SENCHO_EXPERIMENTAL = prev;
+    }
+  });
+
+  it('treats an empty string as false', async () => {
+    const prev = process.env.SENCHO_EXPERIMENTAL;
+    process.env.SENCHO_EXPERIMENTAL = '';
+    try {
+      const res = await request(app).get('/api/meta');
+      expect(res.body.experimental).toBe(false);
+    } finally {
+      if (prev === undefined) delete process.env.SENCHO_EXPERIMENTAL;
+      else process.env.SENCHO_EXPERIMENTAL = prev;
+    }
+  });
+});

backend/src/routes/meta.ts

@@ -22,6 +22,7 @@ metaRouter.get('/meta', (_req: Request, res: Response): void => {
     version: getSenchoVersion(),
     capabilities: getActiveCapabilities(),
     startedAt: processStartedAt,
+    experimental: process.env.SENCHO_EXPERIMENTAL === 'true',
     ...(updateError ? { updateError } : {}),
   });
 });