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/common/mod.rs

@@ -8,3 +8,21 @@ pub mod schema;
 
 /// Standard buffer size for I/O operations (8KB)
 pub const PIPESIZE: usize = 8192;
+
+/// Reads chunks from `reader` until EOF, passing each chunk to `f`.
+///
+/// Uses a fixed PIPESIZE buffer to ensure bounded memory usage.
+pub fn stream_copy<R: std::io::Read + ?Sized>(
+    reader: &mut R,
+    mut f: impl FnMut(&[u8]) -> std::io::Result<()>,
+) -> std::io::Result<()> {
+    let mut buffer = [0u8; PIPESIZE];
+    loop {
+        let n = reader.read(&mut buffer)?;
+        if n == 0 {
+            break;
+        }
+        f(&buffer[..n])?;
+    }
+    Ok(())
+}