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

cmd_chat/server/models.py

@@ -1,6 +1,6 @@
 from dataclasses import dataclass, field
 from uuid import uuid4
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Optional
 
 
@@ -8,7 +8,9 @@ from typing import Optional
 class Message:
     id: str = field(default_factory=lambda: str(uuid4()))
     text: str = ""
-    timestamp: str = field(default_factory=lambda: datetime.utcnow().isoformat())
+    timestamp: str = field(
+        default_factory=lambda: datetime.now(timezone.utc).isoformat()
+    )
     user_ip: str = ""
     username: str = ""
 
@@ -19,13 +21,17 @@ class UserSession:
     ip: str
     username: str = "unknown"
     fernet_key: Optional[bytes] = None
-    created_at: str = field(default_factory=lambda: datetime.utcnow().isoformat())
-    last_activity: str = field(default_factory=lambda: datetime.utcnow().isoformat())
+    created_at: str = field(
+        default_factory=lambda: datetime.now(timezone.utc).isoformat()
+    )
+    last_activity: str = field(
+        default_factory=lambda: datetime.now(timezone.utc).isoformat()
+    )
     active: bool = True
 
     def update_activity(self):
-        self.last_activity = datetime.utcnow().isoformat()
+        self.last_activity = datetime.now(timezone.utc).isoformat()
 
     def is_stale(self, timeout_seconds: int = 3600) -> bool:
         last = datetime.fromisoformat(self.last_activity)
-        return (datetime.utcnow() - last).total_seconds() > timeout_seconds
+        return (datetime.now(timezone.utc) - last).total_seconds() > timeout_seconds