docs: add Exchange Rates event (NIP-33 kind 30078)

[mostronatorcoder]

Overview

Add documentation for the new Exchange Rates publishing feature via Nostr.

Mostro instances can now publish Bitcoin/fiat exchange rates as NIP-33 addressable events (kind 30078) to provide censorship-resistant price data for clients.

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What's New

Exchange Rates Event (Kind 30078)

• Event kind: 30078 (NIP-33 addressable, replaceable)
• d tag: "mostro-rates" (unique identifier)
• Content: Full Yadio API format {"BTC": {"USD": 50000.0, "EUR": 45000.0, ...}}
• Tags:
  • published_at: Unix timestamp (daemon time)
  • source: Rate source (e.g., "yadio")
  • expiration: NIP-40 tag (prevents stale rates)

Configuration

[mostro]
publish_exchange_rates_to_nostr = true  # default
exchange_rates_update_interval_seconds = 300  # default: 5 minutes, min: 60s

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Why Nostr?

In censored regions (Venezuela, Cuba, Iran), HTTP exchange rate APIs are often blocked by governments.

Nostr's decentralized relay network provides:

• ✅ Censorship resistance — impossible to block all relays
• ✅ Zero scaling cost — relays distribute events
• ✅ Geographic distribution — relays worldwide
• ✅ No single point of failure

This aligns with Mostro's mission to provide censorship-resistant peer-to-peer Bitcoin trading.

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Security

⚠️ CRITICAL for clients:

Clients MUST:

1. Verify event.pubkey matches the expected Mostro instance pubkey
2. Verify signature (NIP-01)
3. Check expiration timestamp to avoid stale rates
4. Fall back to HTTP API if Nostr fails

Without pubkey verification, a malicious actor could publish fake rates to manipulate prices.

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Implementation

• Daemon PR: feat: publish exchange rates to Nostr mostro#685
• Client spec PR: feat: add Nostr-based exchange rates spec for v2 app#36
• Issue: Feature: Publish Exchange Rates to Nostr (Censorship-Resistant Alternative to Yadio HTTP API) mostro#684

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Documentation Changes

• Added complete section "Exchange Rates (NIP-33 Kind 30078)" to src/other_events.md
• Includes:
  • Event structure with full example
  • Tag explanations
  • Content format (Yadio structure)
  • Configuration options
  • Client usage guide
  • Security considerations
  • Use case explanation (censorship resistance)

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Testing

• Markdown renders correctly
• JSON examples are valid
• Links are correct
• Formatting is consistent with existing docs

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Related

This feature was motivated by the Venezuela censorship use case where government blocks access to exchange rate APIs, making it impossible for users to see real-time BTC prices for trading.

With Nostr, price data flows freely, enabling peer-to-peer Bitcoin trading even under authoritarian regimes.

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Ready for review! 📖