fix(fleet): drop tier gate from stack and container list endpoints (#1012)

Community tier opened the Fleet Overview drilldown (node card → stack
list → container list) but two read-only metadata endpoints in
fleet.ts kept their requirePaid gate. Stack names still rendered
because they ride on the un-gated /api/fleet/overview payload, but
expanding a stack always re-fetched containers and hit the orphan
gate, surfacing as a "Failed to load containers for X" toast on every
node card on Community.

Drop requirePaid from GET /node/:nodeId/stacks and
GET /node/:nodeId/stacks/:stackName/containers. Both handlers stay
behind authMiddleware, validate inputs, and proxy to the per-node
api_token for remote nodes. Paid mutations (bulk update, scheduled
snapshots, label bulk actions, Fleet Actions cards) keep their gates
in fleetActions.ts. Add positive Community-tier tests in fleet.test.ts
and clear the stale "Requires Skipper or Admiral license" line from
the OpenAPI descriptions.

backend/src/__tests__/fleet.test.ts

@@ -205,6 +205,22 @@ describe('Fleet tier gating', () => {
       .set('Authorization', authHeader);
     expect(res.body.code).not.toBe('PAID_REQUIRED');
   });
+
+  it('GET /api/fleet/node/:nodeId/stacks is accessible on community tier', async () => {
+    mockTier('community');
+    const res = await request(app)
+      .get('/api/fleet/node/1/stacks')
+      .set('Authorization', authHeader);
+    expect(res.body.code).not.toBe('PAID_REQUIRED');
+  });
+
+  it('GET /api/fleet/node/:nodeId/stacks/:stackName/containers is accessible on community tier', async () => {
+    mockTier('community');
+    const res = await request(app)
+      .get('/api/fleet/node/1/stacks/db-compose/containers')
+      .set('Authorization', authHeader);
+    expect(res.body.code).not.toBe('PAID_REQUIRED');
+  });
 });
 
 // ─── Update Endpoint Input Validation ───