Include JSON schema in definition API

We have a JSON schema for corpus definitions, but it can only be accessed through the source code.

Suggestions:
- Include `$schema` property when exporting JSON definitions.
- Allow the frontend to validate JSON uploads against the (current) schema.
- Allow users to inspect the schema. (A URL to the JSON would be sufficient.)

Start by locating the existing corpus-definition JSON schema, the definition API's JSON export, and the frontend's JSON upload handling. Check how exported definitions and uploaded files are processed. Done should include schema exposure in the API, a $schema property on exported definitions, frontend validation against the current schema, and a way for users to inspect the schema.

Written by the indexing model from the issue text.