feat: oss index categories endpoint - #2351

Summary

Adds GET /v1-alpha/oss-index/categories (IN-1415, epic IN-1146). It returns one category group's details and its categories, each with its top collections and top projects.

• categoryGroupSlug picks the group and is required. sort (totalContributors or softwareValue, default totalContributors) orders the categories.
• The route reads the group from category_groups_list first. An unknown slug answers 404 not_found, and the categories pipe is then skipped.
• Categories come from categories_oss_index, with tuples mapped through the shared module from the groups endpoint.
• A malformed row from either pipe answers 503 upstream_unavailable.

Notes

• This pipe's project tuples carry 12 slots without status, so src/lib/oss-index.ts gains CategoryTopProject and its guard and mapper. Both project guards share the leading slots and the v2 health tail. The groups route's shape is unchanged.
• The response is { name, slug, type, categories }, a single group with its child list.
• Stacked on feat: oss index groups endpoint #2350.

Tests

• tests/oss-index-categories.test.ts: both pipe calls and their params, 404 for an unknown slug, 400 for a missing slug or an invalid sort, tuple mapping, 503 for malformed rows, empty categories, and the OpenAPI entry.
• tests/oss-index.test.ts: the 12-slot guard and mapper.

• main
  • feat: oss index categories endpoint 👈
    • feat: oss index collections endpoint #2352