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/core/Cargo.toml

@@ -63,7 +63,7 @@ bytes.workspace = true
 mail-send.workspace = true
 blake3.workspace = true
 uuid.workspace = true
-fjall.workspace = true
+bichon-blob.workspace = true
 tracing-log.workspace = true
 tokio-util.workspace = true
 whichlang = "0.1.1"