feat: auto-generate Nostr keypair during identity creation

Every new identity now gets both Ed25519 (DID) and secp256k1 (Nostr)
keys from creation. The create() method calls create_nostr_key()
automatically, so identity.create RPC always returns nostr_pubkey
and nostr_npub fields.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/archipelago/src/identity_manager.rs

@@ -143,18 +143,15 @@ impl IdentityManager {
             self.set_default(&id).await?;
         }
 
+        // Auto-generate Nostr keypair so every identity has both key types
+        let _ = self.create_nostr_key(&id).await;
+
+        // Re-read to pick up the Nostr keys
+        let record = self.get(&id).await?;
+
         tracing::info!("Created identity '{}' ({})", name, purpose);
 
-        Ok(IdentityRecord {
-            id,
-            name,
-            purpose,
-            pubkey_hex,
-            did,
-            created_at,
-            nostr_pubkey: None,
-            nostr_npub: None,
-        })
+        Ok(record)
     }
 
     /// Get a single identity by ID (without secret key).