feat: add OAuth2 CSRF state management

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

hbd/server/oauth.py

@@ -26,6 +26,29 @@ STATE_TTL = 600  # 10 minutes
 _states: dict[str, float] = {}
 
 
+def make_state() -> str:
+    """Generate a CSRF state token, store it with TTL, and return it."""
+    _purge_states()
+    token = secrets.token_hex(32)
+    _states[token] = time.time() + STATE_TTL
+    return token
+
+
+def validate_state(state: str) -> bool:
+    """Return True if *state* is known and unexpired; always removes it."""
+    expiry = _states.pop(state, None)
+    if expiry is None:
+        return False
+    return time.time() < expiry
+
+
+def _purge_states() -> None:
+    now = time.time()
+    expired = [k for k, exp in list(_states.items()) if exp < now]
+    for k in expired:
+        del _states[k]
+
+
 class OAuthError(Exception):
     """Raised when the OAuth2 flow fails for any reason."""