add compose-based test runner for Python 2.7

Dockerfile.test builds production image with pytest baked in.
compose.test.yml mounts source as volume for fast iteration.
Usage: podman-compose -f compose.test.yml run --rm test

compose.test.yml

@@ -0,0 +1,18 @@
+# PPF test runner (Python 2.7, production deps + pytest)
+#
+# Mounts source and tests as volumes so no rebuild needed between runs.
+#
+# Usage:
+#   podman-compose -f compose.test.yml run --rm test
+#   podman-compose -f compose.test.yml run --rm test python -m pytest tests/test_fetch.py -v
+
+services:
+  test:
+    container_name: ppf-test
+    build:
+      context: .
+      dockerfile: Dockerfile.test
+    volumes:
+      - .:/app:ro,Z
+    working_dir: /app
+    command: python -m pytest tests/ -v --tb=short