Merge pull request #782 from PR0M3TH3AN/codex/add-architecture-overview-to-readme

docs: add Nostr setup guide and architecture overview

docs/nostr_setup.md

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+# Nostr Setup
+
+This guide explains how SeedPass uses the Nostr protocol for encrypted vault backups and how to configure relays.
+
+## Relay Configuration
+
+SeedPass communicates with the Nostr network through a list of relays. You can manage these relays from the CLI:
+
+```bash
+seedpass nostr list-relays      # show configured relays
+seedpass nostr add-relay <url>  # add a relay URL
+seedpass nostr remove-relay <n> # remove relay by index
+```
+
+At least one relay is required for publishing and retrieving backups. Choose relays you trust to remain online and avoid those that charge high fees or aggressively rate‑limit connections.
+
+## Manifest and Delta Events
+
+Backups are published as parameterised replaceable events:
+
+- **Kind 30070 – Manifest:** describes the snapshot and lists chunk IDs. The optional `delta_since` field stores the UNIX timestamp of the latest delta event.
+- **Kind 30071 – Snapshot Chunk:** each 50 KB fragment of the compressed, encrypted vault.
+- **Kind 30072 – Delta:** captures changes since the last snapshot.
+
+When restoring, SeedPass downloads the most recent manifest and applies any newer delta events.
+
+## Troubleshooting
+
+- **No events found:** ensure the relays are reachable and that the correct fingerprint is selected.
+- **Connection failures:** some relays only support WebSocket over TLS; verify you are using `wss://` URLs where required.
+- **Stale data:** if deltas accumulate without a fresh snapshot, run `seedpass nostr sync` to publish an updated snapshot.
+
+Increasing log verbosity with `--verbose` can also help diagnose relay or network issues.