Migrate clip/convert to V3 actions; add background clip support, redirect legacy route; update frontend guide

Dockerfile

@@ -17,8 +17,7 @@ RUN apt-get update; \
 # Install Python requirements
 # This file is now located in the project root
 COPY requirements.txt /tmp/requirements.txt
-RUN --mount=type=cache,target=/root/.cache/pip \
-    pip install -r /tmp/requirements.txt
+RUN pip install -r /tmp/requirements.txt
 
 # Create a reference of actual installed versions
 RUN pip freeze >> /tmp/aether_fastapi_requirements_current.txt