Implement stripe subscription sync

backend/src/billing.rs

@@ -1,6 +1,7 @@
 use anyhow::Result;
 
 use crate::command::Command;
+use crate::models::Activity;
 use crate::query::Query;
 use crate::robot::Robot;
 
@@ -23,6 +24,62 @@ impl Billing {
         }
     }
 
+    pub async fn start(self) {
+        let mut rx = self.command.notify.subscribe();
+
+        loop {
+            match rx.recv().await {
+                Ok(activity) => {
+                    if let Err(e) = self.handle_activity(&activity).await {
+                        tracing::error!(error = %e, "billing handle_activity failed");
+                    }
+                }
+                Err(tokio::sync::broadcast::error::RecvError::Lagged(n)) => {
+                    tracing::warn!(missed = n, "billing lagged");
+                }
+                Err(tokio::sync::broadcast::error::RecvError::Closed) => break,
+            }
+        }
+    }
+
+    async fn handle_activity(&self, activity: &Activity) -> Result<()> {
+        let needs_billing_sync = matches!(
+            activity.activity_type.as_str(),
+            "create_relay" | "update_relay" | "activate_relay" | "deactivate_relay"
+                | "fail_relay_sync" | "complete_relay_sync"
+        );
+
+        if needs_billing_sync {
+            self.sync_relay_subscription_item(activity).await?;
+        }
+
+        Ok(())
+    }
+
+    async fn sync_relay_subscription_item(&self, activity: &Activity) -> Result<()> {
+        let Some(relay) = self.query.get_relay(&activity.resource_id).await? else {
+            return Ok(());
+        };
+
+        let should_delete = !relay.sync_error.is_empty()
+            || relay.synced == 0
+            || relay.plan == "free"
+            || relay.status == "inactive";
+
+        if should_delete {
+            if relay.stripe_subscription_item_id.is_some() {
+                // TODO: Delete subscription item via Stripe API
+                self.command.delete_relay_subscription_item(&relay.id).await?;
+            }
+        } else {
+            // TODO: Create or update subscription item via Stripe API
+            // let stripe_subscription_item_id = ...;
+            // self.command.set_relay_subscription_item(&relay.id, &stripe_subscription_item_id).await?;
+        }
+
+        Ok(())
+    }
+
     pub async fn deactivate_relay(&self, relay_id: &str) -> Result<()> {
         let relay = self
             .query