Introduce listing review and moderation

### Description
Once anyone can submit listings, the directory needs moderation.

### Suggested project states:
- draft
- pending
- published
- rejected
- archived

### Acceptance criteria
- New submissions can require review
- Public endpoints only return published listings
- Maintainers can see submission state
- Moderator actions are auditable

> [!WARNING]
> Blocked by #8

Start by reviewing the work in blocked issue #8, then trace how listings are submitted, returned by public endpoints, viewed by maintainers, and changed by moderators. Done means the five proposed states are supported, unpublished listings stay out of public results, maintainers can see state, and moderator actions are auditable.

Written by the indexing model from the issue text.