feat: add NIP-65 (Relay List Metadata), NIP-25 (Reactions), and Marmot Protocol (kind 443/444/445) support #576

Pledge
N/A — I am planning to implement this myself. @cameri would love your feedback before I start.

Is your feature request related to a problem? Please describe.
Nostream currently lacks support for three NIPs that are either widely used by clients or directly complement the NIP-17+44 work from #458:

• NIP-65 leaves the DM relay routing incomplete clients have no standard way to discover which relay to deliver messages to for a given user
• NIP-25 reactions are expected by virtually every nostr client but not explicitly handled
• Marmot Protocol (kind 443/444/445) is the emerging standard for E2EE group messaging on nostr, and nostream would be one of the first relays to support it

Describe the solution you'd like

NIP-65 - Relay List Metadata (~1 day)

• Accept and store kind:10002 replaceable events
• Advertise support in NIP-11 relay info document
• Completes the DM routing story from feat: implement NIP-17 & NIP-44 v2 for Modern Direct Messages #458

NIP-25 - Reactions (~1.5 days)

• Accept and store kind:7 events
• Validate e and p tag references per spec
• No blocking dependencies

Marmot Protocol relay support (~2 days)

• Accept and store kind:443 (KeyPackages), kind:444 (Welcome Messages), kind:445 (Group Events)
• Relay stays transport-only no MLS logic needed
• Advertise support in NIP-11
• Pairs naturally with NIP-17+44 to form a complete private messaging stack

Describe alternatives you've considered
These could be split into 3 separate issues/PRs happy to do that if you prefer smaller, reviewable chunks.

Additional context
Estimated total: ~5 days. Plan is to open individual PRs for each, this issue is just to align before starting.