debian: Use uv instead of Poetry to export requirements

Replace poetry with uv in the Debian build_virtualenv script for
exporting pip-compatible requirements from the lockfile. This is part
of the migration from Poetry to uv for dependency management (#19572).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

debian/build_virtualenv

@@ -30,15 +30,18 @@ case $(dpkg-architecture -q DEB_HOST_ARCH) in
         ;;
 esac
 
-# Manually install Poetry and export a pip-compatible `requirements.txt`
+# Use uv to export a pip-compatible `requirements.txt` from the lockfile.
+# uv export reads pyproject.toml + uv.lock directly, no venv needed.
 TEMP_VENV="$(mktemp -d)"
 python3 -m venv "$TEMP_VENV"
 source "$TEMP_VENV/bin/activate"
 pip install -U pip
-pip install poetry==2.2.1 poetry-plugin-export==1.9.0
-poetry export \
-    --extras all \
-    --extras test \
+pip install uv
+uv export \
+    --no-emit-project \
+    --frozen \
+    --extra all \
+    --extra test \
     -o exported_requirements.txt
 # Keep journald logging support in the packaged virtualenv when using
 # systemd.journal.JournalHandler in /etc/matrix-synapse/log.yaml.