Fixed chat-initiated research running without the user's database password after the in-memory session-password store expired (24h TTL) or was cleared by a server/container restart while the session cookie was still valid. In that state the research completed (so the user saw results) but every background metric write was silently dropped, leaving the metrics dashboard empty (part of #4457). The chat `send_message` route now applies the same encryption-aware password guard as the direct and follow-up research routes, returning a clear "session expired — log out and log back in" message before any rows are created. Unencrypted installs are unaffected.
