collab: Add GET /billing/usage endpoint (#28832)

This PR adds a `GET /billing/usage` endpoint for retrieving billing
usage to show on the `zed.dev/account` page.

Release Notes:

- N/A

crates/collab/src/db/tables/billing_subscription.rs

@@ -19,6 +19,18 @@ pub struct Model {
     pub created_at: DateTime,
 }
 
+impl Model {
+    pub fn current_period_start_at(&self) -> Option<DateTimeUtc> {
+        let period_start = self.stripe_current_period_start?;
+        chrono::DateTime::from_timestamp(period_start, 0)
+    }
+
+    pub fn current_period_end_at(&self) -> Option<DateTimeUtc> {
+        let period_end = self.stripe_current_period_end?;
+        chrono::DateTime::from_timestamp(period_end, 0)
+    }
+}
+
 #[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]
 pub enum Relation {
     #[sea_orm(