A whoooole lotta 4.0.x fixes.

frontends/web/README_subprocess.md

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+# Subprocess Isolation for Stegasoo WebUI
+
+This update runs encode/decode/compare operations in isolated subprocesses
+to prevent jpegio/scipy crashes from taking down the Flask server.
+
+## Files
+
+- **app.py** - Updated Flask app using subprocess isolation
+- **subprocess_stego.py** - Flask-side wrapper with clean API  
+- **stego_worker.py** - Subprocess script that does actual stegasoo operations
+
+## Setup
+
+1. Place all three files in your `webui/` directory (same level as templates/)
+
+2. Make sure stego_worker.py is executable (optional):
+   ```bash
+   chmod +x stego_worker.py
+   ```
+
+3. Run the Flask app:
+   ```bash
+   python app.py
+   ```
+
+## How It Works
+
+Instead of calling stegasoo functions directly in the Flask process:
+
+```python
+# OLD (crashes could kill Flask)
+result = encode(...)
+```
+
+We now run them in subprocesses:
+
+```python
+# NEW (crashes only kill the subprocess)
+result = subprocess_stego.encode(...)
+```
+
+If jpegio or scipy crashes due to memory corruption, only the subprocess
+dies. Flask logs the error and continues running. The next request spawns
+a fresh subprocess.
+
+## Configuration
+
+In `app.py`, you can adjust the timeout:
+
+```python
+subprocess_stego = SubprocessStego(timeout=180)  # 3 minutes
+```
+
+Larger images may need longer timeouts.
+
+## Troubleshooting
+
+If you see "Worker script not found" errors, make sure `stego_worker.py`
+is in the same directory as `app.py`.
+
+If subprocess operations fail, check the Flask logs for error details.
+The subprocess wrapper captures both stdout and stderr from the worker.