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/update.rs

@@ -6,9 +6,11 @@ use crate::common::PIPESIZE;
 use crate::config;
 use crate::db;
 use crate::services::compression_service::CompressionService;
+use crate::services::meta_service::MetaService;
 use clap::Command;
 use log::debug;
 use rusqlite::Connection;
+use std::sync::{Arc, Mutex};
 
 /// Handles the update mode: modifies tags and metadata for an existing item by ID.
 ///
@@ -93,6 +95,13 @@ pub fn mode_update(
         db::set_item_tags(conn, item.clone(), tags)?;
     }
 
+    // Run meta plugins if --meta-plugin flags are provided
+    let plugin_names = settings.meta_plugins_names();
+    if !plugin_names.is_empty() {
+        debug!("UPDATE: Running meta plugins: {:?}", plugin_names);
+        run_meta_plugins_on_item(conn, cmd, settings, &data_path, &item, item_id)?;
+    }
+
     // Backfill size if not set
     let mut updated_item = item.clone();
     if item.size.is_none() {
@@ -169,3 +178,59 @@ fn compute_item_size(data_path: &Path, item: &db::Item) -> Option<i64> {
 
     Some(total_bytes)
 }
+
+/// Runs meta plugins on an existing item's content and stores the results.
+fn run_meta_plugins_on_item(
+    conn: &mut Connection,
+    cmd: &mut Command,
+    settings: &config::Settings,
+    data_path: &Path,
+    item: &db::Item,
+    item_id: i64,
+) -> Result<()> {
+    let mut item_path = data_path.to_path_buf();
+    item_path.push(item_id.to_string());
+
+    if !item_path.exists() {
+        debug!("UPDATE: Content file not found: {item_path:?}");
+        return Ok(());
+    }
+
+    // Collect metadata in memory
+    let collected_meta: Arc<Mutex<Vec<(String, String)>>> = Arc::new(Mutex::new(Vec::new()));
+    let collector = collected_meta.clone();
+    let save_meta: crate::meta_plugin::SaveMetaFn =
+        Arc::new(Mutex::new(move |name: &str, value: &str| {
+            if let Ok(mut v) = collector.lock() {
+                v.push((name.to_string(), value.to_string()));
+            }
+        }));
+
+    let meta_service = MetaService::new(save_meta);
+    let mut plugins = meta_service.get_plugins(cmd, settings);
+
+    if plugins.is_empty() {
+        return Ok(());
+    }
+
+    let compression_service = CompressionService::new();
+    let mut reader = compression_service.stream_item_content(item_path, &item.compression)?;
+
+    meta_service.initialize_plugins(&mut plugins);
+
+    crate::common::stream_copy(&mut reader, |chunk| {
+        meta_service.process_chunk(&mut plugins, chunk);
+        Ok(())
+    })?;
+
+    meta_service.finalize_plugins(&mut plugins);
+
+    // Write collected plugin metadata to DB
+    if let Ok(entries) = collected_meta.lock() {
+        for (name, value) in entries.iter() {
+            db::add_meta(conn, item_id, name, value)?;
+        }
+    }
+
+    Ok(())
+}