feat(core): send uploaded files with a chat message

[davemooreuws]

Second of the chat attachments PRs, stacked on #393: sending uploads with a message. The web app doesn't send them yet.

• POST /chats/:chatId/messages takes attachments, the ids of the sender's unsent uploads. A message can be files only.
• Files are attached in the same transaction that posts the message, so the announced message carries them. A file that isn't one of the sender's unsent uploads fails the send with a 400 and posts nothing.
• Messages gain a file part, read from thread_file, in the order the sender listed them, kept as each file's position.
• Bots see each attachment as a line in the sender's message, naming the file and the tool that reads it. Reading images and recognising text from content comes next.

Part of SUGA-1825

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Status	Name	Latest Commit	Updated (UTC)
✅ Deployment successful! View logs	sugabots-website	e2f62cc	Oct 16 2026, 06:45 AM