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/conftest.py

@@ -0,0 +1,41 @@
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+import uuid
+import pytest
+from sanic_testing import TestManager
+from sanic import Sanic
+from sanic_ext import Extend
+from cryptography.fernet import Fernet
+
+from cmd_chat.server.managers import ConnectionManager
+from cmd_chat.server.stores import MessageStore, UserSessionStore
+from cmd_chat.server.srp_auth import SRPAuthManager
+from cmd_chat.server.routes import register_routes
+
+
+@pytest.fixture
+def app():
+    name = f"test-{uuid.uuid4().hex[:8]}"
+    
+    app = Sanic(name)
+    Extend(app)
+
+    app.ctx.message_store = MessageStore()
+    app.ctx.session_store = UserSessionStore()
+    app.ctx.connection_manager = ConnectionManager()
+    app.ctx.srp_manager = SRPAuthManager("testpassword")
+    app.ctx.fernet_key = Fernet.generate_key()
+    app.ctx.cleanup_task = None
+
+    register_routes(app)
+    TestManager(app)
+    
+    return app
+
+
+@pytest.fixture
+def test_client(app):
+    return app.test_client