Add list collections query to GraphQL

[shangyian]

Summary

This PR adds a listCollections GraphQL query and enables searching collections by fragment as well as by the user who created it. It implements several optimizations, including eager loading for user data, SQL subqueries for node counts, and DataLoader-based batching for node loading.

Example:

{
  listCollections(fragment: "repair", createdBy: "dj") {
    name
    description
    nodeCount
  }
}

Test Plan

Added this new test suite:

uv pytest tests/api/graphql/collections_test.py -v

• PR has an associated issue: #
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

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