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/save.rs

@@ -84,7 +84,10 @@ pub fn mode(
             compressor.write_all(&buffer[..n])?;
         }
 
-        // Finalize compression (flushes any buffered compressed data)
+        // Explicitly flush and finalize compression before dropping.
+        // LZ4 FrameEncoder buffers data internally; without explicit flush,
+        // only the frame header (7 bytes) gets written to the pipe.
+        compressor.flush()?;
         drop(compressor);
 
         // Pipe writer is now dropped (inside compressor), signaling EOF to streamer
@@ -147,6 +150,17 @@ pub fn mode(
         uncompressed_size.to_string(),
     );
 
+    // Record client compression type so the client can decompress on retrieval.
+    // When compress=false, the server stores the blob as-is with compression=None.
+    // Without this metadata, the client would get compressed bytes back but think
+    // they're uncompressed.
+    if !matches!(compression_type, CompressionType::None) {
+        local_metadata.insert(
+            "_client_compression".to_string(),
+            compression_type.to_string(),
+        );
+    }
+
     // Add hostname
     if let Ok(hostname) = gethostname::gethostname().into_string() {
         local_metadata.insert("hostname".to_string(), hostname.clone());