Add indexed_by=maybe to cards

[robzolkos]

Summary

This adds indexed_by=maybe to GET /:account_slug/cards so API clients can ask the cards endpoint for awaiting-triage / "Maybe?" cards.

What wasn't possible before

Before this change, GET /:account_slug/cards supported indexed_by=closed and indexed_by=not_now, but there was no equivalent way to ask that same endpoint for awaiting-triage cards.

Clients that wanted "Maybe?" cards had to either:

• fetch a broader /cards result set and filter client-side for cards with no workflow column, or
• switch to the board-scoped stream endpoint instead of using /cards

That made the cards endpoint asymmetric: Done and Not Now were directly filterable there, but Maybe? was not.

What this PR adds

• support indexed_by=maybe on GET /:account_slug/cards
• route that filter to awaiting-triage cards
• document the new indexed_by value
• add model and controller coverage

Example

• GET /:account_slug/cards?indexed_by=maybe

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[flavorjones]

@robzolkos Looks good! Needs to be rebased, though.

[robzolkos]

@flavorjones rebased and we have a transient test issue which is fixed by #2838 Once that one is merged (if ok) I'll rebase again - test should pass.