refactor: decouple meta plugins from DB via SaveMetaFn callback, extract shared utilities

- Add SaveMetaFn callback pattern: meta plugins receive a closure instead of
  &Connection, enabling the same plugin code to work in local, client, and
  server contexts (collect-to-Vec, collect-to-HashMap, or direct DB write)
- Client save now runs meta plugins locally during streaming (smart client
  sets meta=false, server skips its own plugins)
- Add POST /api/item/{id}/update endpoint for re-running plugins on stored
  content without downloading compressed data
- Add client update mode (--update with --meta-plugin flags)
- Extract shared utilities: stream_copy, print_serialized, build_path_table,
  ensure_default_tag to reduce duplication across modes
- Add upsert_tag for idempotent tag addition (INSERT OR IGNORE)
- Add warn logging on save_meta lock failure in BaseMetaPlugin and MetaService

src/modes/list.rs

@@ -306,15 +306,7 @@ fn show_list_structured(
         list_items.push(list_item);
     }
 
-    match output_format {
-        OutputFormat::Json => {
-            println!("{}", serde_json::to_string_pretty(&list_items)?);
-        }
-        OutputFormat::Yaml => {
-            println!("{}", serde_yaml::to_string(&list_items)?);
-        }
-        OutputFormat::Table => unreachable!(),
-    }
+    crate::modes::common::print_serialized(&list_items, &output_format)?;
 
     Ok(())
 }