Update Render build configuration: fix deps, force py3.11, add build script

backend/render-build.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+python -m pip install --upgrade pip setuptools wheel
+python -m pip install --retries 10 --timeout 120 -r requirements.txt
+
+# Download required NLTK and spaCy models during build phase
+python -m spacy download en_core_web_sm
+python -m nltk.downloader punkt_tab stopwords averaged_perceptron_tagger