Refactor /schedule endpoint

[Gerzer]

Currently, the /schedule endpoint merely redirects to a schedule.json static file that contains all known schedule data. We should refactor the existing endpoint into two new endpoints:

• /schedules for reading an array of all known schedules
• /schedules/active for reading the currently active schedule (with an invariant that in the source of truth for all schedules, none may overlap in the axis of time)

Both of these endpoints should accept only GET requests. They also should return their respective responses directly without any redirection, preferably by implementing a proper data model in the SQLite database.

This will be a breaking change to the public API, so we’ll need to increment the API version number. We should try to batch this change with other upcoming breaking changes to minimize instability.