feat(auth): server-side sessions and current password for account changes (FLAW-30, FLAW-31)

- Every login is a session (device, IP, last use); the token's jti is its ID.
  Sessions end on logout, per device from the profile, or after 30 idle days.
- Tokens older than a day are renewed via the X-Session-Token header.
- Changing password or login e-mail needs the current password; a new
  password logs out all other devices and mails the old address.
- Demo reset keeps the demo visitors' sessions.
- revoked_tokens is replaced by sessions (migration 4); everyone is logged
  out once after the upgrade.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/realtime.go

@@ -19,7 +19,7 @@ func (s *Server) authenticateUserFromReq(r *http.Request) string {
 			return ""
 		}
 		u, err := s.store.GetUserByID(tk.userID)
-		if err != nil || u == nil || !u.IsActive || u.TokenVersion != tk.tokenVersion {
+		if err != nil || u == nil || !u.IsActive || u.TokenVersion != tk.tokenVersion || !s.store.SessionActive(tk.sessionID, u.ID) {
 			return ""
 		}
 		return u.ID