Return aggregated per-project snapshots for the fleet overview.

Response shape mirrors :func:bernstein.core.fleet.web.api_projects:

.. code-block:: json

{
  "projects": [ProjectSnapshot, ...],
  "errors": [],
  "stub": true|false,
  "hint": "Run `bernstein fleet --web` for the real aggregator."
}

stub: true means the operator UI is talking to a single-project server that has no fleet aggregator wired in; the projects list is empty in that case so the SPA can render the empty-state.