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/common.rs

@@ -1,3 +1,4 @@
+use crate::common::status::PathInfo;
 use crate::compression_engine::CompressionType;
 /// Common utilities shared across different modes in the Keep application.
 ///
@@ -564,6 +565,28 @@ pub fn apply_color(mut cell: Cell, color: &config::TableColor, is_foreground: bo
     cell
 }
 
+/// Ensures tags has at least one entry, adding "none" if empty.
+pub fn ensure_default_tag(tags: &mut Vec<String>) {
+    if tags.is_empty() {
+        tags.push("none".to_string());
+    }
+}
+
+/// Prints a serializable value in JSON or YAML format based on output format.
+///
+/// Only handles Json and Yaml variants; Table should be handled separately.
+pub fn print_serialized<T: serde::Serialize>(
+    value: &T,
+    format: &OutputFormat,
+) -> anyhow::Result<()> {
+    match format {
+        OutputFormat::Json => println!("{}", serde_json::to_string_pretty(value)?),
+        OutputFormat::Yaml => println!("{}", serde_yaml::to_string(value)?),
+        OutputFormat::Table => unreachable!(),
+    }
+    Ok(())
+}
+
 /// Applies config TableAttribute to a comfy-table Cell.
 pub fn apply_table_attribute(mut cell: Cell, attribute: &config::TableAttribute) -> Cell {
     match attribute {
@@ -580,3 +603,18 @@ pub fn apply_table_attribute(mut cell: Cell, attribute: &config::TableAttribute)
 
     cell
 }
+
+/// Builds a table showing data and database path information.
+pub fn build_path_table(path_info: &PathInfo, table_config: &config::TableConfig) -> Table {
+    let mut path_table = create_table_with_config(table_config);
+
+    path_table.set_header(vec![
+        Cell::new("Type").add_attribute(Attribute::Bold),
+        Cell::new("Path").add_attribute(Attribute::Bold),
+    ]);
+
+    path_table.add_row(vec!["Data", &path_info.data]);
+    path_table.add_row(vec!["Database", &path_info.database]);
+
+    path_table
+}