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/Cargo.toml

@@ -17,4 +17,11 @@ serde_json.workspace = true
 
 itertools.workspace = true
 snafu.workspace = true
-bichon-memdb.workspace = true
+bichon-memdb.workspace = true
+fjall.workspace = true
+mail-parser.workspace = true
+bytes.workspace = true
+uuid.workspace = true
+tantivy = { version = "0.26.1", features = ["zstd-compression", "quickwit"] }
+chrono.workspace = true
+tracing.workspace = true