feat: implement a basic backend (#1)

Reviewed-on: https://git.reya.su/reya/signed/pulls/1

crates/signed_nostr/src/update.rs

@@ -0,0 +1,32 @@
+use nostr_sdk::prelude::*;
+
+/// A lightweight "something changed" signal for the UI.
+///
+/// Heavy data stays in the database; consumers re-query on receipt.
+#[derive(Debug, Clone)]
+pub struct Update {
+    pub kind: Kind,
+    /// First `a` tag value of the event, if any (e.g. the repository coordinate).
+    pub coordinate: Option<String>,
+    pub author: PublicKey,
+    pub event_id: EventId,
+}
+
+impl Update {
+    /// Build an update from a received event.
+    pub fn from_event(event: &Event) -> Self {
+        let coordinate = event
+            .tags
+            .iter()
+            .find(|t| t.kind() == "a")
+            .and_then(|t| t.content())
+            .map(str::to_owned);
+
+        Self {
+            kind: event.kind,
+            coordinate,
+            author: event.pubkey,
+            event_id: event.id,
+        }
+    }
+}