Install an Endpoint with no local catalog

[rbardaji]

Closes #214.

The lightest Endpoint there is — nothing installed, nothing stored locally, every optional integration off — could not be chosen. The installer's local-catalog question had four answers and all four configured a catalog, and no combination of environment variables produced a working Endpoint without one.

What this adds

LOCAL_CATALOG_BACKEND=none is now a deployment that stores nothing locally: no MongoDB, no CKAN. It authenticates users, searches the platform's global catalog, serves its UI and reports to the Federation.

The installer offers it as the first answer and as the default for --backend:

  Which local catalog should this Endpoint use?
    1) None — nothing is stored locally (quickest)
    2) MongoDB, installed alongside the Endpoint
    3) MongoDB, one I already have
    4) CKAN, installed by this script (takes several minutes)
    5) CKAN, one I already have
  Choice [1]:

It renders LOCAL_CATALOG_BACKEND=none with CKAN_LOCAL_ENABLED=False and blanks CKAN_URL, CKAN_API_KEY and MONGODB_CONNECTION_STRING, so nothing is left pointing at a service that was not installed. Running the installer again with --backend mongodb or --backend ckan adds a catalog later.

Why the API needed changing too

Switching the local catalog off was not enough on its own:

• The readiness probe reported the catalog as disabled only for the CKAN backend. With any other backend it probed the catalog even though CKAN_LOCAL_ENABLED=False had already unmounted the local catalog routes and made /search?server=local answer 400 — so an Endpoint configured deliberately without a usable catalog answered HTTP 503 and looked broken to any orchestrator.
• Catalog statistics are collected inside the same try as the rest of the metrics payload. Asking for a repository that does not exist left the payload empty and skipped the POST, so the Endpoint would have stopped reporting to the Federation altogether rather than reporting without dataset counts.
• The routes that write to a local catalog were mounted on the master switch alone, without asking whether a backend provided one.

Verified

• Full suite in the built image: 1199 passed. Installer render tests: 16 passed. black --check . and flake8 clean.
• A real no-catalog install: the installer rendered the configuration, the Endpoint came up, /health and /ready answered 200 with local_catalog: {status: disabled, backend: none}, the UI served, /search?server=global answered 200, /search?server=local answered 400 with "Local CKAN is disabled and cannot be used", the registration/update/delete/resource routes were absent from the OpenAPI schema, and metrics posted to the Federation successfully.

Backwards compatibility

ckan and mongodb behave exactly as before. Two changes affect existing setups:

• A deployment running with CKAN_LOCAL_ENABLED=False and a MongoDB backend was answering 503 on /ready and now answers 200 with the catalog reported disabled.
• Unattended installer runs that did not pass --backend used to get MongoDB and now get no catalog; add --backend mongodb to keep the previous result.

[rbardaji]

Also closes #216: CI was failing on every branch, this one included, because mcp 2.0.0 broke fastapi-mcp 0.4.0 and FastApiMCP(app) runs at import time — a container built today did not start at all. Pinned to mcp<2.0.0, which is a prerequisite for releasing this at all, since the release image is built fresh.