proxy: compress captures with zstd (#668)

The previous captures were saved uncompressed in memory. In agentic
workflows there can be many turns with each request containing the
previous context in the body with a lot of redundant data. Use zstd to
compress the request and response data before keeping a copy of memory.

Results: 

- Average Percentage Saved: 73.19%
- Average Compression Factor: ~6.77:1

proxy/proxymanager_api.go

@@ -290,11 +290,26 @@ func (pm *ProxyManager) apiGetCapture(c *gin.Context) {
 		return
 	}
 
-	capture := pm.metricsMonitor.getCaptureByID(id)
-	if capture == nil {
+	data, exists := pm.metricsMonitor.getCompressedBytes(id)
+	if !exists {
 		c.JSON(http.StatusNotFound, gin.H{"error": "capture not found"})
 		return
 	}
 
-	c.JSON(http.StatusOK, capture)
+	c.Header("Vary", "Accept-Encoding")
+
+	// ¯\_(ツ)_/¯ quality weights are too fancy for us anyway
+	hasZstd := strings.Contains(c.GetHeader("Accept-Encoding"), "zstd")
+
+	if hasZstd {
+		c.Header("Content-Encoding", "zstd")
+		c.Data(http.StatusOK, "application/json", data)
+	} else {
+		decompressed, err := decompressCapture(data)
+		if err != nil {
+			c.JSON(http.StatusInternalServerError, gin.H{"error": "failed to decompress capture"})
+			return
+		}
+		c.Data(http.StatusOK, "application/json", decompressed)
+	}
 }