chore: prevent duplicate Lightning charges by adding durable invoice-level NWC payment guard (#51)

Reviewed-on: coracle/caravel#51
Co-authored-by: userAdityaa <aditya.chaudhary1558@gmail.com>
Co-committed-by: userAdityaa <aditya.chaudhary1558@gmail.com>

backend/src/query.rs

@@ -161,6 +161,16 @@ impl Query {
         Ok(row)
     }
 
+    pub async fn get_invoice_nwc_payment_state(&self, invoice_id: &str) -> Result<Option<String>> {
+        let state = sqlx::query_scalar::<_, String>(
+            "SELECT state FROM invoice_nwc_payment WHERE invoice_id = ?",
+        )
+        .bind(invoice_id)
+        .fetch_optional(&self.pool)
+        .await?;
+        Ok(state)
+    }
+
     pub async fn has_active_paid_relays(&self, tenant_id: &str) -> Result<bool> {
         let plans = sqlx::query_scalar::<_, String>(
             "SELECT plan FROM relay WHERE tenant = ? AND status = 'active'",