feat: add NIP-25 (Reactions) support #578

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

Is your feature request related to a problem? Please describe.
Nostream does not explicitly handle NIP-25 (Reactions). Reactions are expected by virtually every nostr client and have no blocking dependencies in nostream.

Describe the solution you'd like

• Accept and store kind:7 events
• Validate e and p tag references per spec
• Advertise support in NIP-11 relay info document

Describe alternatives you've considered
No alternatives - NIP-25 is the standard for reactions across the nostr ecosystem.

Additional context
Estimated: ~1.5 days.