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/db.rs

@@ -337,6 +337,18 @@ pub fn add_tag(conn: &Connection, item_id: i64, tag_name: &str) -> Result<()> {
     insert_tag(conn, tag)
 }
 
+/// Adds a tag to an item, ignoring if the tag already exists.
+///
+/// Uses `INSERT OR IGNORE` to make the operation idempotent.
+pub fn upsert_tag(conn: &Connection, item_id: i64, tag_name: &str) -> Result<()> {
+    debug!("DB: Upserting tag: item={item_id}, tag={tag_name}");
+    conn.execute(
+        "INSERT OR IGNORE INTO tags (id, name) VALUES (?1, ?2)",
+        params![item_id, tag_name],
+    )?;
+    Ok(())
+}
+
 /// Adds metadata to an item.
 ///
 /// Inserts a new metadata entry in the `metas` table.