fiat invoice to Lightning msat conversion by applying real-time BTC FX quotes (#7)

Co-authored-by: userAdityaa <aditya.chaudhary1558@gmail.com>
Co-committed-by: userAdityaa <aditya.chaudhary1558@gmail.com>

backend/tests/btc_quote_stub.rs

@@ -0,0 +1,31 @@
+use axum::{Json, Router, routing::get};
+use backend::billing::{fetch_btc_spot_price_from_base, fiat_minor_to_msats_from_quote};
+
+#[tokio::test]
+async fn quote_endpoint_can_be_stubbed_deterministically() {
+    async fn spot() -> Json<serde_json::Value> {
+        Json(serde_json::json!({ "data": { "amount": "50000.00" } }))
+    }
+
+    let app = Router::new().route("/v2/prices/BTC-USD/spot", get(spot));
+
+    let listener = tokio::net::TcpListener::bind("127.0.0.1:0")
+        .await
+        .expect("bind test server");
+    let addr = listener.local_addr().expect("get local addr");
+    tokio::spawn(async move {
+        axum::serve(listener, app).await.expect("serve quote stub");
+    });
+
+    let client = reqwest::Client::new();
+    let base = format!("http://{addr}/v2/prices");
+    let btc_price = fetch_btc_spot_price_from_base(&client, &base, "USD")
+        .await
+        .expect("fetch stubbed quote");
+
+    assert_eq!(btc_price, 50_000.0);
+
+    let msats = fiat_minor_to_msats_from_quote(100, "USD", btc_price)
+        .expect("convert quoted fiat amount");
+    assert_eq!(msats, 2_000_000);
+}