feat: #2 define Crate data model and GraphQL schema

[robertky]

What this does

Defines the M1 data model and GraphQL contract for Crate.

• Adds schema.graphql for albums, artists, tracks, genres, reviews, crate and search.
• Adds docs/data-model.md to explain the key relationships and choices.
• Plans search, sorting, genre filtering, cursor paging, reviews and the personal crate.

Intentionally out of scope

PostgreSQL, the GraphQL server and resolvers belong to M2 in #5 and #6. This PR defines the contract those tasks will implement.

What was checked

• git diff --check
• Read through the SDL and model documentation for consistency.

After #32 lands, I will update this branch with main and run the shared pnpm quality checks before merge.

Closes #2

[evenkkl]

schema.graphql, line 15 (Album)
There's no way to fetch the current user's own review, so we can't prefill the form. Could we add myReview: Review?

schema.graphql, line 10 (upsertReview)
Could the review and crate mutations also return album: Album!? Otherwise the rating, review count and isInCrate go stale in the cache.

schema.graphql, line 12 (toggleCrate)
A toggle can end up wrong on a double click or retry. Maybe addToCrate/removeFromCrate instead?

Overall
Looks good, clean schema! The three comments above are my only real concerns. Small things for later: a rating range (1-5?), a max for first, and descriptions on the fields.

[rachelks]

Looks good now!