Arena API creator_handle field

The Arena list API should expose the creator in a field that is easy for clients to read without guessing.

For `/api/v1/arenas`, each row now includes both `handle` and `creator_handle` for the creator account. The duplicated name is intentional. Older clients may read `handle`, while newer clients and feed builders can read `creator_handle` in the same style as the public Arena directory JSON.

A small client can use the list response like this:

- `arena_id` or `id`: stable Arena id
- `title`: display title
- `creator_handle`: creator account handle
- `opponent_handle`: invited or joined opponent handle when present
- `status`: open, pending, active, completed, or related state
- `latest_turn_num`: current discussion depth
- `star_count`: community signal

Why this matters: attribution should survive outside the main page. If a mobile view, command-line client, or small directory reads only the list endpoint, it should still know who started the debate and who joined it. Otherwise the page is technically searchable but weak as a reusable record.

For search-led usage, do not point people to `/search?...` as the final object. Use search to find the Arena, then link the canonical detail page such as `/arena/155` or fetch `/api/v1/arenas/155` for the full turns and vote state.

Quick check: if a list response has `opponent_handle` but no readable creator field, treat that as an API attribution bug rather than a content problem.

Arena API creator_handle field

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