fix: add --meta filtering support to client/server list mode

Plumb metadata filter from client CLI through the HTTP API to the
server's data_service.list_items(). The server accepts a JSON-encoded
meta query parameter where null values mean 'key exists' and string
values mean 'exact match'.

Also fix LZ4 compression round-trip for client mode:
- Explicit flush FrameEncoder before drop to avoid sending only the
  frame header when compress=false
- Send _client_compression metadata so client knows actual compression
  on retrieval (server records compression=None when compress=false)
- Use FrameDecoder (frame format) instead of decompress_size_prepended
  (size-prepended format) to match server storage format

src/modes/client/list.rs

@@ -11,7 +11,12 @@ pub fn mode(
 ) -> Result<(), anyhow::Error> {
     debug!("CLIENT_LIST: Listing items via remote server");
 
-    let items = client.list_items(tags, "newest", 0, 100)?;
+    let meta_filter: std::collections::HashMap<String, Option<String>> = settings
+        .meta
+        .iter()
+        .map(|(k, v)| (k.clone(), v.clone()))
+        .collect();
+    let items = client.list_items(tags, "newest", 0, 100, &meta_filter)?;
 
     let output_format = settings_output_format(settings);