fix(db): isolate usage metrics advisory-lock test on scratch DB (#3670)

## What Problem This Solves

`test_usage_metrics_lock_has_single_owner_and_releases_on_drop`
hardcodes the **production** advisory lock key (`0x4255_5A5A_4D45_5452`)
on the shared `TEST_DATABASE_URL`. Postgres advisory locks are
per-database, so any live `buzz-relay` pointed at the same DB holds that
key and the test fails (or races the relay tick). Diagnosis time was
burned during #3268 verification, including near-misses on live dev
relays. Fixes #3619.

## Why This Change Was Made

Preferred fix from the issue: run the test on a private scratch DB via
existing `create_scratch_db` / `drop_scratch_db` (same pattern as
replica-routing fixtures). Keep the production lock key so the test
still documents the real constant, without colliding with a running
relay.

## User Impact

- Local `cargo test -p buzz-db -- --ignored` no longer fails when a dev
relay is running against the shared test DB
- Safer: no temptation to `pg_terminate_backend` a live relay to "fix"
the test

## Evidence

- Code review of fixture isolation
- Pattern matches existing `create_scratch_db` usage in this file
- Test remains `#[ignore = "requires Postgres"]` (same as before)

## Related

- Issue: #3619
- None found among open PRs for this exact fix

Signed-off-by: NanoRisk6 <aidashtherapy@gmail.com>

crates/buzz-db/src/lib.rs

@@ -5780,15 +5780,21 @@ mod tests {
     #[tokio::test]
     #[ignore = "requires Postgres"]
     async fn test_usage_metrics_lock_has_single_owner_and_releases_on_drop() {
-        let database_url =
-            std::env::var("TEST_DATABASE_URL").unwrap_or_else(|_| TEST_DB_URL.into());
-        let pool = PgPoolOptions::new()
-            .max_connections(2)
-            .connect(&database_url)
+        // Use a private scratch database — not the shared TEST_DATABASE_URL.
+        // Postgres advisory locks are per-database; hardcoding the production
+        // USAGE_METRICS_LOCK_KEY (0x4255_5A5A_4D45_5452) on the shared test DB
+        // races any live buzz-relay on the same database (see #3619).
+        let admin_url = std::env::var("TEST_DATABASE_URL").unwrap_or_else(|_| TEST_DB_URL.into());
+        let admin = PgPoolOptions::new()
+            .max_connections(1)
+            .connect(&admin_url)
             .await
-            .expect("connect to test DB");
+            .expect("connect admin to create scratch db");
+        let (pool, scratch_name) = create_scratch_db(&admin, "usage_metrics_lock").await;
         let first = Db::from_pool(pool.clone());
-        let second = Db::from_pool(pool);
+        let second = Db::from_pool(pool.clone());
+        // Same key as production (`buzz-relay` USAGE_METRICS_LOCK_KEY) — safe here
+        // because the scratch DB is empty of other holders.
         let key = 0x4255_5A5A_4D45_5452;
 
         let mut leader = first
@@ -5815,6 +5821,11 @@ mod tests {
                 .is_some(),
             "dropping the detached session releases its advisory lock"
         );
+
+        // Release any remaining session state before DROP DATABASE.
+        drop(first);
+        drop(second);
+        drop_scratch_db(&admin, pool, &scratch_name).await;
     }
 
     #[tokio::test]