fix (billing): use fx quote for stripe to lightning

backend/src/api.rs

@@ -800,8 +800,9 @@ async fn get_invoice_bolt11(
     }
 
     let amount_due = invoice["amount_due"].as_i64().unwrap_or(0);
+    let currency = invoice["currency"].as_str().unwrap_or("usd");
 
-    match state.api.billing.create_bolt11(amount_due).await {
+    match state.api.billing.create_bolt11(amount_due, currency).await {
         Ok(bolt11) => Ok(ok(StatusCode::OK, serde_json::json!({ "bolt11": bolt11 }))),
         Err(e) => Ok(err(
             StatusCode::INTERNAL_SERVER_ERROR,

backend/src/billing.rs

@@ -13,6 +13,7 @@ use crate::robot::Robot;
 type HmacSha256 = Hmac<Sha256>;
 
 const STRIPE_API: &str = "https://api.stripe.com/v1";
+const COINBASE_SPOT_API: &str = "https://api.coinbase.com/v2/prices";
 const WEBHOOK_TOLERANCE_SECS: i64 = 300;
 
 #[derive(serde::Deserialize)]
@@ -27,6 +28,16 @@ struct StripeEventData {
     object: serde_json::Value,
 }
 
+#[derive(serde::Deserialize)]
+struct CoinbaseSpotPriceResponse {
+    data: CoinbaseSpotPriceData,
+}
+
+#[derive(serde::Deserialize)]
+struct CoinbaseSpotPriceData {
+    amount: String,
+}
+
 #[derive(Clone)]
 pub struct Billing {
     nwc_url: String,
@@ -186,8 +197,9 @@ impl Billing {
             "invoice.created" => {
                 let customer = obj["customer"].as_str().unwrap_or_default();
                 let amount_due = obj["amount_due"].as_i64().unwrap_or(0);
+                let currency = obj["currency"].as_str().unwrap_or("usd");
                 let invoice_id = obj["id"].as_str().unwrap_or_default();
-                self.handle_invoice_created(customer, amount_due, invoice_id)
+                self.handle_invoice_created(customer, amount_due, currency, invoice_id)
                     .await?;
             }
             "invoice.paid" => {
@@ -253,6 +265,7 @@ impl Billing {
         &self,
         stripe_customer_id: &str,
         amount_due: i64,
+        currency: &str,
         invoice_id: &str,
     ) -> Result<()> {
         if amount_due == 0 {
@@ -269,7 +282,10 @@ impl Billing {
 
         // 1. NWC auto-pay: if the tenant has a nwc_url
         if !tenant.nwc_url.is_empty() {
-            match self.nwc_pay_invoice(amount_due, &tenant.nwc_url).await {
+            match self
+                .nwc_pay_invoice(amount_due, currency, &tenant.nwc_url)
+                .await
+            {
                 Ok(()) => {
                     self.stripe_pay_invoice_out_of_band(invoice_id).await?;
                     self.command
@@ -467,8 +483,8 @@ impl Billing {
         Ok(body)
     }
 
-    pub async fn create_bolt11(&self, amount_due_cents: i64) -> Result<String> {
+    pub async fn create_bolt11(&self, amount_due_minor: i64, currency: &str) -> Result<String> {
-        let amount_msats = (amount_due_cents as u64) * 1000; // placeholder conversion
+        let amount_msats = self.fiat_minor_to_msats(amount_due_minor, currency).await?;
 
         let system_uri: NostrWalletConnectURI = self.nwc_url.parse()
             .map_err(|_| anyhow!("invalid system NWC URL"))?;
@@ -642,11 +658,13 @@ impl Billing {
 
     // --- NWC helpers ---
 
-    async fn nwc_pay_invoice(&self, amount_due_cents: i64, tenant_nwc_url: &str) -> Result<()> {
+    async fn nwc_pay_invoice(
-        // Convert USD cents to millisatoshis (approximate: 1 sat ≈ variable USD)
+        &self,
-        // amount_due is in cents from Stripe. We create a Lightning invoice for the exact amount.
+        amount_due_minor: i64,
-        // The NWC make_invoice amount is in millisatoshis.
+        currency: &str,
-        let amount_msats = (amount_due_cents as u64) * 1000; // placeholder conversion, actual rate would come from exchange
+        tenant_nwc_url: &str,
+    ) -> Result<()> {
+        let amount_msats = self.fiat_minor_to_msats(amount_due_minor, currency).await?;
 
         // Create a bolt11 invoice using the system wallet (self.nwc_url)
         let system_uri: NostrWalletConnectURI = self.nwc_url.parse()
@@ -683,4 +701,97 @@ impl Billing {
 
         Ok(())
     }
+
+    async fn fiat_minor_to_msats(&self, amount_due_minor: i64, currency: &str) -> Result<u64> {
+        let normalized_currency = currency.to_uppercase();
+        let btc_price = self.fetch_btc_spot_price(&normalized_currency).await?;
+        Self::fiat_minor_to_msats_from_quote(amount_due_minor, &normalized_currency, btc_price)
+    }
+
+    async fn fetch_btc_spot_price(&self, currency: &str) -> Result<f64> {
+        let pair = format!("BTC-{currency}");
+        let url = format!("{COINBASE_SPOT_API}/{pair}/spot");
+        let resp = self.http.get(url).send().await?;
+        let body: CoinbaseSpotPriceResponse = resp.error_for_status()?.json().await?;
+
+        let amount = body
+            .data
+            .amount
+            .parse::<f64>()
+            .map_err(|e| anyhow!("invalid BTC spot quote for {currency}: {e}"))?;
+
+        if amount <= 0.0 {
+            return Err(anyhow!("invalid non-positive BTC spot quote for {currency}"));
+        }
+
+        Ok(amount)
+    }
+
+    fn fiat_minor_to_msats_from_quote(
+        amount_due_minor: i64,
+        currency: &str,
+        btc_price_in_fiat: f64,
+    ) -> Result<u64> {
+        if amount_due_minor <= 0 {
+            return Err(anyhow!("amount_due must be positive"));
+        }
+
+        if btc_price_in_fiat <= 0.0 {
+            return Err(anyhow!("btc_price_in_fiat must be positive"));
+        }
+
+        let exponent = Self::currency_minor_exponent(currency)?;
+        let divisor = 10_f64.powi(exponent as i32);
+        let amount_fiat = (amount_due_minor as f64) / divisor;
+        let amount_btc = amount_fiat / btc_price_in_fiat;
+        let raw_msats = amount_btc * 100_000_000_000.0;
+        // Guard against tiny floating point artifacts at integer boundaries.
+        let amount_msats = if (raw_msats - raw_msats.round()).abs() < 1e-6 {
+            raw_msats.round()
+        } else {
+            raw_msats.ceil()
+        };
+
+        if !amount_msats.is_finite() || amount_msats <= 0.0 || amount_msats > u64::MAX as f64 {
+            return Err(anyhow!("calculated msat amount is out of bounds"));
+        }
+
+        Ok(amount_msats as u64)
+    }
+
+    fn currency_minor_exponent(currency: &str) -> Result<u8> {
+        let normalized = currency.to_uppercase();
+        let exponent = match normalized.as_str() {
+            // Zero-decimal currencies in Stripe.
+            "BIF" | "CLP" | "DJF" | "GNF" | "JPY" | "KMF" | "KRW" | "MGA" | "PYG"
+            | "RWF" | "UGX" | "VND" | "VUV" | "XAF" | "XOF" | "XPF" => 0,
+            // Three-decimal currencies in Stripe.
+            "BHD" | "JOD" | "KWD" | "OMR" | "TND" => 3,
+            _ if normalized.chars().all(|c| c.is_ascii_alphabetic()) && normalized.len() == 3 => {
+                2
+            }
+            _ => return Err(anyhow!("invalid currency code: {currency}")),
+        };
+
+        Ok(exponent)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Billing;
+
+    #[test]
+    fn converts_usd_minor_units_with_quote() {
+        let msats = Billing::fiat_minor_to_msats_from_quote(100, "usd", 100_000.0)
+            .expect("conversion should succeed");
+        assert_eq!(msats, 1_000_000);
+    }
+
+    #[test]
+    fn converts_zero_decimal_currency_with_quote() {
+        let msats = Billing::fiat_minor_to_msats_from_quote(100, "jpy", 10_000_000.0)
+            .expect("conversion should succeed");
+        assert_eq!(msats, 1_000_000);
+    }
 }