Add token refresh logic

[Hozyne-OpenBak]

Part of #2

Auto-refresh expired OAuth tokens.

Tasks:

• Check token expiry before each request
• Refresh if <5 min remaining
• Update stored tokens after refresh
• Handle refresh failure (revoke + re-auth required)

Logic:

if (Date.now() >= token.expires_at - 300000) {
  // Refresh needed
  const refreshed = await provider.refresh(token.refresh_token);
  await storage.update(refreshed);
}