add db, user registering into db, format

src/db.ml

@@ -0,0 +1,13 @@
+let db_root = App.data_dir
+
+let () =
+  match Bos.OS.Dir.create (Fpath.v db_root) with
+  | Ok true -> Dream.log "created %s" db_root
+  | Ok false -> Dream.log "%s already exists" db_root
+  | Error (`Msg _) ->
+    Dream.warning (fun log -> log "error when creating %s" db_root)
+
+let db = Filename.concat db_root "permap.db"
+
+let with_db ?mode ?mutex ?cache ?vfs ?timeout f =
+  Sqlite3_utils.with_db ?mode ?mutex ?cache ?vfs ?timeout db f