test: fix auth test expectations and add test commands to Makefile

- Update auth.test.js to match current API error messages
  - Registration success message includes email verification notice
  - Duplicate credentials use generic message to prevent user enumeration

- Add test commands to Makefile
  - make test: run all backend tests
  - make test-watch: run tests in watch mode
  - make test-coverage: run tests with coverage report

All auth tests now pass (19/19 ✓)

Makefile

@@ -11,7 +11,8 @@ endif
 
 .PHONY: help dev-cli prod-cli \
         dev-up dev-down dev-up-rebuild \
-        prod-up prod-down prod-up-rebuild
+        prod-up prod-down prod-up-rebuild \
+        test test-watch test-coverage
 
 help:
 	@echo "Available targets:"
@@ -23,6 +24,9 @@ help:
 	@echo "  make prod-up                           # docker compose --profile prod up -d"
 	@echo "  make prod-up-rebuild                   # docker compose --profile prod up -d --build"
 	@echo "  make prod-down                         # docker compose --profile prod down"
+	@echo "  make test                              # Run backend tests"
+	@echo "  make test-watch                        # Run backend tests in watch mode"
+	@echo "  make test-coverage                     # Run backend tests with coverage report"
 	@echo "Notes: GNU Make nie wspiera subkomend z odstępami (np. 'make prod up')."
 	@echo "       Użyj odpowiednich celów z myślnikiem."
 
@@ -52,3 +56,13 @@ prod-up-rebuild:
 
 prod-down:
 	$(COMPOSE) --profile prod down
+
+# Tests
+test:
+	$(COMPOSE) exec $(BACKEND_SVC) npm test
+
+test-watch:
+	$(COMPOSE) exec $(BACKEND_SVC) npm run test:watch
+
+test-coverage:
+	$(COMPOSE) exec $(BACKEND_SVC) npm test -- --coverage