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/diff.rs

@@ -3,7 +3,6 @@
 /// This module provides functionality for comparing two items and displaying their
 /// differences using external diff tools. Decompressed content is streamed to diff
 /// via pipes and /dev/fd file descriptors — no temporary files are created.
-use crate::common::PIPESIZE;
 use crate::config;
 use crate::services::compression_service::CompressionService;
 use crate::services::item_service::ItemService;
@@ -118,17 +117,11 @@ fn spawn_writer_thread(
 
         // Convert OwnedFd to File — safe, takes ownership, closes on drop
         let mut writer = std::fs::File::from(write_fd);
-        let mut buf = [0u8; PIPESIZE];
-        loop {
-            match reader.read(&mut buf) {
-                Ok(0) => break,
-                Ok(n) => {
-                    use std::io::Write;
-                    writer.write_all(&buf[..n])?;
-                }
-                Err(e) => return Err(anyhow::anyhow!("Error reading item {item_id}: {e}")),
-            }
-        }
+        crate::common::stream_copy(&mut reader, |chunk| {
+            use std::io::Write;
+            writer.write_all(chunk)
+        })
+        .map_err(|e| anyhow::anyhow!("Error reading item {item_id}: {e}"))?;
         // writer dropped here, closing write_fd → diff sees EOF
         Ok(())
     })