Finish the API endpoint for cover upload

src/storage.cr

@@ -2,6 +2,7 @@ require "sqlite3"
 require "crypto/bcrypt"
 require "uuid"
 require "base64"
+require "./util"
 
 def hash_password(pw)
   Crypto::Bcrypt::Password.create(pw).to_s
@@ -11,10 +12,6 @@ def verify_password(hash, pw)
   (Crypto::Bcrypt::Password.new hash).verify pw
 end
 
-def random_str
-  UUID.random.to_s.gsub "-", ""
-end
-
 class Storage
   def initialize(@path : String, @logger : Logger)
     dir = File.dirname path