[brendanreardon]

This Pull Request makes the API easier to understand and use from its documentation alone, and keeps that documentation from drifting out of date. Specifically:

• The interactive docs at api.moalmanac.org now list every filter each endpoint accepts, with descriptions and examples, plus a short guide to how filters combine (different filters narrow results; repeating a filter broadens them).
• Every endpoint also shows what its response looks like, using moalmanac-db's own schemas and a real example record, so the docs update automatically whenever the database does.

Several small changes were made too:

• response timestamps are now valid
• /about reports the correct record count
• agent_type no longer can be used to filter /documents, /indications, and /statements

It also updates the API for moalmanac-db's rename of identification_number to regulatory_identifier and imports moalmanac-db's code directly (the submodule folder is now moalmanac_db). Finally, the full API description is saved as docs/openapi.json for the documentation site, and a new GitHub Actions check runs lint, the test suite, and a check that this file is current on every pull request.