Feature: support large outgoing attachments via R2-backed links

## Summary

Add support for large outgoing attachments by uploading them to R2 and inserting a secure download link into the email, similar to Gmail’s large-file link fallback.

## Problem

Large attachments can hit email provider limits and Worker request body limits. Instead of forcing every file into the email payload, oversized files should be stored in R2 and shared as links.

## Proposed behavior

- Small files continue as normal email attachments.
- Files above a configurable limit are uploaded to R2.
- The email body gets a secure download link.
- The UI shows filename, size, upload state, and link status.
- The agent can reference attachment metadata without loading the full file.

## Suggested config

```env
MAX_INLINE_ATTACHMENT_MB=25
MAX_R2_LINK_ATTACHMENT_MB=500
R2_ATTACHMENT_LINK_TTL_DAYS=30
````

## Implementation idea

Store files in R2 under a scoped key like:

```txt
mailboxes/{mailboxId}/outgoing-attachments/{draftId}/{attachmentId}
```

Store metadata with the draft/message:

* filename
* content type
* size
* R2 key
* expiry
* download token/hash

Downloads should go through the Worker using an unguessable or signed URL, instead of exposing raw R2 URLs.

For very large files, multipart/chunked upload should be supported.

## Acceptance criteria

* [ ] Large attachments can be uploaded to R2.
* [ ] Emails include secure download links.
* [ ] Inline vs R2-link thresholds are configurable.
* [ ] Attachment metadata is stored with drafts/sent messages.
* [ ] Agent tools can see linked attachment metadata.
* [ ] Links are private, expiring, or at least unguessable.

## References

* Cloudflare R2 limits: [https://developers.cloudflare.com/r2/platform/limits/](https://developers.cloudflare.com/r2/platform/limits/)
* R2 multipart uploads: [https://developers.cloudflare.com/r2/api/workers/workers-multipart-usage/](https://developers.cloudflare.com/r2/api/workers/workers-multipart-usage/)
* Workers limits: [https://developers.cloudflare.com/workers/platform/limits/](https://developers.cloudflare.com/workers/platform/limits/)
* Gmail attachment limits: [https://support.google.com/mail/answer/6584](https://support.google.com/mail/answer/6584)
* Gmail Drive link behavior: [https://support.google.com/mail/answer/2487407](https://support.google.com/mail/answer/2487407)