(core) Monthly per-site API limit for free plans

Summary:
Free plans had a daily limit per document. Now they have one limit for whole
site, 3000 calls per month, and the daily one is removed from those plans.

- Limits are only checked for requests made with an api key. Before, rest
  endpoints counted browser calls like downloads too. With a much smaller
  limit that would be visible.
- MCP reports a used limit to the model, not as a transport HTTP error.
- Billing page and new "Personal site" page in account settings show what
  is left. Personal sites have no billing page, so they need own place.
- Owner or billing manager gets a banner when the limit is close or over.

Test Plan: Added tests

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D5045