test(db): derive the latest migration in the 0075 invite upgrade test

Rebasing onto main put this branch's `channels.auto_join` migration at
0076, after main's 0074 (user deletion fences) and 0075 (invite
revocation). `migration_schema_0075_adds_invite_columns_without_rewriting_history`
asserted that a full migrate ends at version 75, so it failed once 0076
existed. It now reads the latest version from the migrations directory,
as `thread_window_postgres_tests` already does, so the next migration does
not break it again.

The renumbering itself (0074 -> 0076, with the migrator pin test resolved
to main's contiguity loop at 76) was applied in the replayed "add a
channel auto_join setting" commit. Every SELECT that feeds
`row_to_channel_record` already selects `auto_join`, and the three
auto-join call sites sit in the same arms after main's invite-revocation
rewrite, so nothing else needed to change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Matt Toohey <contact@matttoohey.com>

crates/buzz-db/src/store/relay_invite/upgrade_postgres_tests.rs

@@ -506,12 +506,21 @@ async fn apply_pgschema_to_existing_database(pool: &PgPool) {
 async fn migration_schema_0075_adds_invite_columns_without_rewriting_history() {
     let (pool, cases) = legacy_fixture().await;
     crate::migration::run_migrations(&pool).await.unwrap();
+    let latest = sqlx::migrate::Migrator::new(
+        std::path::Path::new(env!("CARGO_MANIFEST_DIR")).join("../../migrations"),
+    )
+    .await
+    .unwrap()
+    .iter()
+    .map(|m| m.version)
+    .max()
+    .unwrap();
     assert_eq!(
         sqlx::query_scalar::<_, i64>("SELECT max(version) FROM _sqlx_migrations WHERE success")
             .fetch_one(&pool)
             .await
             .unwrap(),
-        75
+        latest
     );
     assert_admission_column(&pool).await;
     assert_join_role_preserved(&pool, cases[0].community).await;