fix: silent relay state drift when activity bus drops events (#53)

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

backend/src/billing.rs

@@ -120,6 +120,10 @@ impl Billing {
     pub async fn start(self) {
         let mut rx = self.command.notify.subscribe();
 
+        if let Err(error) = self.reconcile_relay_subscriptions("startup").await {
+            tracing::error!(error = %error, "failed to reconcile relay billing state on startup");
+        }
+
         loop {
             match rx.recv().await {
                 Ok(activity) => {
@@ -129,12 +133,39 @@ impl Billing {
                 }
                 Err(tokio::sync::broadcast::error::RecvError::Lagged(n)) => {
                     tracing::warn!(missed = n, "billing lagged");
+
+                    if let Err(error) = self.reconcile_relay_subscriptions("lagged").await {
+                        tracing::error!(error = %error, "failed to reconcile relay billing state after lag");
+                    }
                 }
                 Err(tokio::sync::broadcast::error::RecvError::Closed) => break,
             }
         }
     }
 
+    async fn reconcile_relay_subscriptions(&self, source: &str) -> Result<()> {
+        let relays = self.query.list_relays().await?;
+
+        if relays.is_empty() {
+            return Ok(());
+        }
+
+        tracing::info!(source, relay_count = relays.len(), "reconciling relay billing state");
+
+        for relay in relays {
+            if let Err(error) = self.sync_relay_subscription_for_relay(&relay).await {
+                tracing::error!(
+                    source,
+                    relay = %relay.id,
+                    error = %error,
+                    "failed to reconcile relay billing state"
+                );
+            }
+        }
+
+        Ok(())
+    }
+
     async fn handle_activity(&self, activity: &Activity) -> Result<()> {
         let needs_billing_sync = matches!(
             activity.activity_type.as_str(),
@@ -158,6 +189,10 @@ impl Billing {
             return Ok(());
         };
 
+        self.sync_relay_subscription_for_relay(&relay).await
+    }
+
+    async fn sync_relay_subscription_for_relay(&self, relay: &Relay) -> Result<()> {
         let Some(tenant) = self.query.get_tenant(&relay.tenant).await? else {
             return Ok(());
         };