- use a single Engine instance for email + attachment blobs
- add delete_batch, gc_if_needed, background flush to blob crate
- fix Entry.raw_size storing compressed length instead of original
- move fjall-dependent migration code from core to admin crate
- add STORAGE_VERSION file for layout version detection
- Make MBOX/PST upload size limits configurable via SETTINGS
(bichon_web_mbox_upload_limit_mb defaults to 1 GB,
bichon_web_pst_upload_limit_mb defaults to 2 GB)
Switched to a personal fork of async-imap to enable a newer version
of imap-proto, addressing dependency constraints and improving
compatibility with recent parser changes.