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.

docs/openapi.yaml

@@ -1888,7 +1888,7 @@ paths:
       operationId: getFleetNodeStacks
       tags: [Fleet]
       summary: List stacks on a fleet node
-      description: Returns stack names from a specific fleet node. Requires Skipper or Admiral license.
+      description: Returns stack names from a specific fleet node.
       parameters:
         - name: nodeId
           in: path
@@ -1928,7 +1928,7 @@ paths:
       operationId: getFleetNodeStackContainers
       tags: [Fleet]
       summary: List containers in a fleet node stack
-      description: Returns containers for a specific stack on a specific fleet node. Requires Skipper or Admiral license.
+      description: Returns containers for a specific stack on a specific fleet node.
       parameters:
         - name: nodeId
           in: path