feat: add SRP authentication, improve security

- Replace RSA key exchange with SRP (Secure Remote Password)
- Password never transmitted over network
- Add unit tests for endpoints
- Fix datetime.UTC compatibility for Python < 3.11
- Fix logger.exception usage
- Update README with new auth flow diagram

tests/test_health.py

@@ -0,0 +1,17 @@
+# tests/test_health.py
+import pytest
+
+
+class TestHealth:
+    """Тесты health endpoint"""
+
+    def test_health_ok(self, test_client):
+        """GET /health возвращает статус"""
+        _, response = test_client.get("/health")
+
+        assert response.status == 200
+        data = response.json
+        assert data["status"] == "ok"
+        assert "messages" in data
+        assert "users" in data
+        assert "timestamp" in data