Attachment size preflight + streaming for large files

[longevityboris]

Problem

• Attachments are fully loaded into memory and base64-encoded (33% inflation)
• No size validation against Resend's 40 MB limit
• Fixed 30s timeout for downloads — large files may fail
• Received attachment URLs are temporary (undocumented expiry)

Proposal

1. Check attachment size before encoding; warn/fail if >30 MB (40 MB after base64)
2. Make download timeout configurable: --timeout <seconds>
3. Stream large downloads instead of buffering entirely in memory
4. Document attachment URL expiry in help text

Priority: LOW