v0.5.0

What's new

• L402 paywall support — client.L402.CreateChallenge(), client.L402.Verify(), client.L402.Pay()
• Get/watch by payment hash — GetByHash() and WatchByHash() on Invoices and Payments
• Removed client.Keys.List() — server endpoint removed (key listing is a local CLI operation)

Breaking changes

• Keys.List() removed
• APIKey type removed