Add documentation for diagram POST caching feature

Documentation/DIAGRAM_CACHING.md

@@ -0,0 +1,105 @@
+# Diagram POST Caching Implementation
+
+This feature replaces the URL-encoded GET approach for diagram generation with POST requests and local filesystem caching.
+
+## Changes Overview
+
+### New Files
+- `diagramm_proxy/__init__.py` - Module initialization
+- `diagramm_proxy/diagram_cache.py` - Caching logic and POST request handling
+- `abschnitte/management/commands/clear_diagram_cache.py` - Management command for cache clearing
+
+### Modified Files
+- `abschnitte/utils.py` - Updated `render_textabschnitte()` to use caching
+- `.gitignore` - Added cache directory exclusion
+
+## Configuration Required
+
+Add to your Django settings file (e.g., `VorgabenUI/settings.py`):
+
+```python
+# Diagram cache settings
+DIAGRAM_CACHE_DIR = 'diagram_cache'  # relative to MEDIA_ROOT
+
+# Ensure MEDIA_ROOT and MEDIA_URL are configured
+MEDIA_ROOT = os.path.join(BASE_DIR, 'media')
+MEDIA_URL = '/media/'
+```
+
+### URL Configuration
+
+Ensure media files are served in development. In your main `urls.py`:
+
+```python
+from django.conf import settings
+from django.conf.urls.static import static
+
+# ... existing urlpatterns ...
+
+# Serve media files in development
+if settings.DEBUG:
+    urlpatterns += static(settings.MEDIA_URL, document_root=settings.MEDIA_ROOT)
+```
+
+## How It Works
+
+1. When a diagram is rendered, the system computes a SHA256 hash of the diagram content
+2. It checks if a cached SVG exists for that hash
+3. If cached: serves the existing file
+4. If not cached: POSTs content to Kroki server, saves the response, and serves it
+5. Diagrams are served from `MEDIA_URL/diagram_cache/{type}/{hash}.svg`
+
+## Benefits
+
+- **No URL length limitations** - Content is POSTed instead of URL-encoded
+- **Improved performance** - Cached diagrams are served directly from filesystem
+- **Reduced server load** - Kroki server is only called once per unique diagram
+- **Persistent cache** - Survives application restarts
+- **Better error handling** - Graceful fallback on generation failures
+
+## Usage
+
+### Viewing Diagrams
+No changes required - diagrams will be automatically cached on first render.
+
+### Clearing Cache
+
+Clear all cached diagrams:
+```bash
+python manage.py clear_diagram_cache
+```
+
+Clear diagrams of a specific type:
+```bash
+python manage.py clear_diagram_cache --type plantuml
+python manage.py clear_diagram_cache --type mermaid
+```
+
+## Testing
+
+1. Create or view a page with diagrams
+2. Verify diagrams render correctly
+3. Check that `media/diagram_cache/` directory is created with cached SVGs
+4. Refresh the page - second load should be faster (cache hit)
+5. Check logs for cache hit/miss messages
+6. Test cache clearing command
+
+## Migration Notes
+
+- Existing diagrams will be regenerated on first view after deployment
+- The old URL-based approach is completely replaced
+- No database migrations needed
+- Ensure `requests` library is installed (already in requirements.txt)
+
+## Troubleshooting
+
+### Diagrams not rendering
+- Check that MEDIA_ROOT and MEDIA_URL are configured correctly
+- Verify Kroki server is accessible at `http://svckroki:8000`
+- Check application logs for error messages
+- Ensure media directory is writable
+
+### Cache not working
+- Verify Django storage configuration
+- Check file permissions on media/diagram_cache directory
+- Review logs for cache-related errors