feat: replace fjall with bichon-blob for blob storage

- 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

crates/admin/src/main.rs

@@ -21,8 +21,10 @@ use dialoguer::{theme::ColorfulTheme, Select};
 
 use crate::{migrate::handle_migration, reset::handle_reset_password};
 
+pub mod legacy;
 pub mod meta;
 pub mod migrate;
+pub mod migrate_store;
 pub mod reset;