feat: make digest_value optional and add digest display

src/db.rs

@@ -32,9 +32,6 @@ lazy_static! {
                  FOREIGN KEY(id) REFERENCES items(id) ON DELETE CASCADE,
                  PRIMARY KEY(id, name));"
         ),
-        M::up(
-            "ALTER TABLE items ALTER COLUMN compression SET DEFAULT 'none';"
-        ),
         M::up(
             "ALTER TABLE items ADD COLUMN digest_type TEXT NOT NULL DEFAULT 'none';"
         ),
@@ -51,7 +48,7 @@ pub struct Item {
     pub size: Option<i64>,
     pub compression: String,
     pub digest_type: String,
-    pub digest_value: String,
+    pub digest_value: Option<String>,
 }
 
 #[derive(Debug, Clone)]