Implement exporter using nocap from nostr-watch along with tests

.env.example

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+export RELAYS=wss://offchain.pub,wss://nostr.bitcoiner.social
+# 0 = run probes on each /metrics scrape; set >0 for periodic background probing.
+# Example: 300000 = every 5 minutes.
+export PROBE_INTERVAL_MS=300000
+export PROBE_TIMEOUT_MS=10000
+export LISTEN_ADDR=0.0.0.0
+export PORT=9464
+export LOG_LEVEL=info
+
+# Optional write confirmation probe settings:
+export WRITE_CHECK_ENABLED=true
+export WRITE_CHECK_KIND=30078
+# WRITE_CHECK_PRIVKEY accepts either:
+# - nsec1... string
+# - 64-char hex private key
+# If omitted or invalid, relay-exporter generates an ephemeral key at runtime.
+#export WRITE_CHECK_PRIVKEY=nsec1xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx