feat: add build query resolver and corresponding tests

[ludvigz]

Purpose

The GraphQL schema defines Query.build(id: ID!): Build, but no resolver was implemented, so requesting a single published build by ID always returned null. This PR adds the missing resolver.

Changes

• Add the build(id) resolver in backend/server/src/resolvers.ts and register it under Query.
• Validate the ID
• Fetch the build with a parameterized query, returns null if it doesent exist.
• Add resolver tests in backend/server/src/test/resolvers.test.ts.
• Add test tooling to the server package.
• Limit tsconfig.json to src and exclude src/test so tests aren't compiled into dist.

Related issues

Closes #95

Verification

Automated checks, run from backend/server:

• npm test: 10 tests pass. They cover successful retrieval with a parameterized query, a nonexistent ID returning null, rejection of invalid IDs without querying the database, and Build.parts.

Manual testing:

Ran against the database, see screenshot

Screenshots

Checklist

• I have reviewed my own changes.
• The changes meet the linked issue's acceptance criteria, where applicable.
• I have verified the changes and recorded the results above.
• I have added or updated tests where needed.
• I have updated documentation where needed.
• I have included screenshots for visual changes, or marked them not applicable.

[wilhelsr]

LGTM! The resolver is well implemented, and the tests cover the key scenarios. Thanks for the fix! Approved