Add unit test suite with pytest configuration

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Makefile

@@ -1,4 +1,4 @@
-.PHONY: deploy build up down restart logs migrate migrate-new frontend
+.PHONY: deploy build up down restart logs migrate migrate-new frontend test
 
 # Build and deploy
 deploy: build up
@@ -29,6 +29,13 @@ migrate-new:
 migrate-history:
 	docker compose exec raggr aerich history
 
+# Tests
+test:
+	pytest tests/ -v
+
+test-cov:
+	pytest tests/ -v --cov
+
 # Frontend
 frontend:
 	cd raggr-frontend && yarn install && yarn build