Add waitlist summary API

crates/collab/src/db_tests.rs

@@ -966,8 +966,8 @@ async fn test_signups() {
         db.create_signup(Signup {
             email_address: format!("person-{i}@example.com"),
             platform_mac: true,
-            platform_linux: true,
-            platform_windows: false,
+            platform_linux: i % 2 == 0,
+            platform_windows: i % 4 == 0,
             editor_features: vec!["speed".into()],
             programming_languages: vec!["rust".into(), "c".into()],
         })
@@ -975,6 +975,16 @@ async fn test_signups() {
         .unwrap();
     }
 
+    assert_eq!(
+        db.get_waitlist_summary().await.unwrap(),
+        WaitlistSummary {
+            count: 8,
+            mac_count: 8,
+            linux_count: 4,
+            windows_count: 2,
+        }
+    );
+
     // retrieve the next batch of signup emails to send
     let signups_batch1 = db.get_unsent_invites(3).await.unwrap();
     let addresses = signups_batch1
@@ -1016,6 +1026,17 @@ async fn test_signups() {
         ]
     );
 
+    // the sent invites are excluded from the summary.
+    assert_eq!(
+        db.get_waitlist_summary().await.unwrap(),
+        WaitlistSummary {
+            count: 5,
+            mac_count: 5,
+            linux_count: 2,
+            windows_count: 1,
+        }
+    );
+
     // user completes the signup process by providing their
     // github account.
     let (user_id, inviter_id) = db