feat: add Marmot Protocol relay support (kind 443/444/445) #579

Pledge
N/A - I am planning to implement this myself.

Is your feature request related to a problem? Please describe.
Nostream has no support for the Marmot Protocol, the emerging standard for E2EE group messaging on nostr. Adding relay support would make nostream one of the first relays to explicitly support it, and it pairs naturally with the NIP-17+44 work from #458.

Describe the solution you'd like

• Accept and store kind:443 (KeyPackages) and kind:445 (Group Events); Welcome Messages (kind:444) are delivered via existing kind:1059 gift wrap infrastructure (MIP-02) and blocked from direct publishing
• Relay stays transport-only, no MLS logic needed
• Advertise support in NIP-11 relay info document

Describe alternatives you've considered
No alternatives - the relay layer for Marmot is intentionally kept simple by design, all MLS complexity lives on the client side.

Additional context
Estimated: ~2 days.