Add missing fields to the GraphQL schema

[anettkva]

The database has some data that backend/src/schema.graphql doesn't expose yet. Add it, and update docs/API.md to match.

Done when:

• Gymnast has a list of the elements and vaults she can do or is practicing, with mutations to add and remove items
• Routine has createdAt and updatedAt
• CatalogItem shows whether an item has been removed from the Code of Points, so the UI can warn about it in routines and element lists
• docs/API.md describes the new fields and mutations, including limits and error codes where relevant