feat: dashboard screenshot upload & retention (last 20 per client)

- Listener: subscribe to dashboard topic, forward screenshots to API
- API: store latest + last 20 timestamped screenshots per client, auto-delete older files
- Docs: updated README, TECH-CHANGELOG, and copilot-instructions for screenshot upload and retention policy

server/routes/clients.py

@@ -273,6 +273,85 @@ def restart_client(uuid):
         return jsonify({"error": f"Failed to send MQTT message: {str(e)}"}), 500
 
 
+@clients_bp.route("/<uuid>/screenshot", methods=["POST"])
+def upload_screenshot(uuid):
+    """
+    Route to receive and store a screenshot from a client.
+    Expected payload: base64-encoded image data in JSON or binary image data.
+    Screenshots are stored as {uuid}.jpg in the screenshots folder.
+    Keeps last 20 screenshots per client (auto-cleanup).
+    """
+    import os
+    import base64
+    import glob
+    from datetime import datetime
+
+    session = Session()
+    client = session.query(Client).filter_by(uuid=uuid).first()
+    if not client:
+        session.close()
+        return jsonify({"error": "Client nicht gefunden"}), 404
+    session.close()
+
+    try:
+        # Handle JSON payload with base64-encoded image
+        if request.is_json:
+            data = request.get_json()
+            if "image" not in data:
+                return jsonify({"error": "Missing 'image' field in JSON payload"}), 400
+            
+            # Decode base64 image
+            image_data = base64.b64decode(data["image"])
+        else:
+            # Handle raw binary image data
+            image_data = request.get_data()
+
+        if not image_data:
+            return jsonify({"error": "No image data received"}), 400
+
+        # Ensure screenshots directory exists
+        screenshots_dir = os.path.join(os.path.dirname(__file__), "..", "screenshots")
+        os.makedirs(screenshots_dir, exist_ok=True)
+
+        # Store screenshot with timestamp to track latest
+        timestamp = datetime.now().strftime("%Y%m%d_%H%M%S")
+        filename = f"{uuid}_{timestamp}.jpg"
+        filepath = os.path.join(screenshots_dir, filename)
+
+        with open(filepath, "wb") as f:
+            f.write(image_data)
+
+        # Also create/update a symlink or copy to {uuid}.jpg for easy retrieval
+        latest_filepath = os.path.join(screenshots_dir, f"{uuid}.jpg")
+        with open(latest_filepath, "wb") as f:
+            f.write(image_data)
+
+        # Cleanup: keep only last 20 timestamped screenshots per client
+        pattern = os.path.join(screenshots_dir, f"{uuid}_*.jpg")
+        existing_screenshots = sorted(glob.glob(pattern))
+        
+        # Keep last 20, delete older ones
+        max_screenshots = 20
+        if len(existing_screenshots) > max_screenshots:
+            for old_file in existing_screenshots[:-max_screenshots]:
+                try:
+                    os.remove(old_file)
+                except Exception as cleanup_error:
+                    # Log but don't fail the request if cleanup fails
+                    import logging
+                    logging.warning(f"Failed to cleanup old screenshot {old_file}: {cleanup_error}")
+
+        return jsonify({
+            "success": True,
+            "message": f"Screenshot received for client {uuid}",
+            "filename": filename,
+            "size": len(image_data)
+        }), 200
+
+    except Exception as e:
+        return jsonify({"error": f"Failed to process screenshot: {str(e)}"}), 500
+
+
 @clients_bp.route("/<uuid>", methods=["DELETE"])
 @admin_or_higher
 def delete_client(uuid):