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/client.rs

@@ -226,6 +226,7 @@ impl KeepClient {
         order: &str,
         start: u64,
         count: u64,
+        meta: &HashMap<String, Option<String>>,
     ) -> Result<Vec<ItemInfo>, CoreError> {
         #[derive(serde::Deserialize)]
         struct ApiResponse {
@@ -239,6 +240,12 @@ impl KeepClient {
         if !tags.is_empty() {
             params.push(("tags".to_string(), tags.join(",")));
         }
+        if !meta.is_empty() {
+            let meta_json = serde_json::to_string(meta).map_err(|e| {
+                CoreError::Other(anyhow::anyhow!("Failed to serialize meta filter: {}", e))
+            })?;
+            params.push(("meta".to_string(), meta_json));
+        }
 
         let param_refs: Vec<(&str, &str)> = params
             .iter()