fix: change self reference to mutable in save_meta and remove unused imports

Co-authored-by: aider (openai/andrew/openrouter/qwen/qwen3-coder) <aider@aider.chat>

src/meta_plugin.rs

@@ -64,7 +64,7 @@ pub trait MetaPlugin {
     }
     
     // Save metadata to database
-    fn save_meta(&self, conn: &Connection, item_id: i64, value: String) -> Result<()> {
+    fn save_meta(&mut self, conn: &Connection, item_id: i64, value: String) -> Result<()> {
         let meta = crate::db::Meta {
             id: item_id,
             name: self.meta_name(),