Bug: Atom feed thumbnails generate malformed XML

[SimoneMariaRomeo]

Problem

AtomFeedBuilder._build_entry() emits malformed XML when an entry includes thumbnail_url.

Current output shape:

<media:thumbnail url="https://example.com/thumb.jpg/>

The closing quote is missing before />, so the generated Atom feed cannot be parsed by XML clients.

Reproduction

1. Create an AtomFeedBuilder.
2. Add an entry with thumbnail_url="https://example.com/thumb.jpg".
3. Build the feed and parse it as XML.

Expected

<media:thumbnail url="https://example.com/thumb.jpg"/>

Impact

Any BoTTube Atom feed entry with a thumbnail can break /api/feed/atom consumers.

Bounty identity

Wallet/miner ID: SimoneMariaRomeo-codex-earner