fix(join): concurrent first joins no longer 500 on the QR-scan burst

`Event::find_or_create` was check-then-insert against a UNIQUE slug, and its only
callers are `/join` and `/admin/login` — both of which run before the row exists,
at the single most concurrent moment the app ever sees: the QR code goes up and
every phone in the room posts `/join` within the same second. All of them miss
the SELECT, all of them INSERT, one wins, and the rest get a bare unique
violation surfaced as a 500 on the very first screen of the event. There is no
retry on that path and nothing in the UI explains it.

In the documented timeline the host's T-5 admin login creates the row first, so
the blast radius is small — but it is one `down -v` or one `EVENT_SLUG` edit away
from being live on the night.

`ON CONFLICT (slug) DO UPDATE SET slug = EXCLUDED.slug` — a deliberate no-op
write, because `DO NOTHING` returns no row on conflict and would put the loser
back at square one. It touches only `slug`, so `name`, `export_epoch` and the
lock/release timestamps are never disturbed by a late arrival; a test pins that.
The read fast-path stays, so every join after the first is still a plain SELECT
and takes no row lock.

Tests live in `src/` rather than `tests/` because the crate is a binary and the
function is not importable from an integration test.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/src/models/event.rs

@@ -32,14 +32,32 @@ impl Event {
             .await
     }
 
+    /// Insert the event, or return the existing row if another request won the race.
+    ///
+    /// `ON CONFLICT`, not a bare INSERT. `slug` is UNIQUE (migration 002), and the only callers are
+    /// `/join` and `/admin/login` — both of which run before the row exists, at the one moment the
+    /// app is most concurrent: the QR code goes up and every phone in the room posts `/join` within
+    /// the same second. A check-then-insert loses that race by construction, and the losers got a
+    /// bare unique violation surfaced as a 500 on the very first screen of the event.
+    ///
+    /// `DO UPDATE SET slug = EXCLUDED.slug` is a deliberate no-op write: `DO NOTHING` returns no
+    /// row on conflict, which would put the loser right back at square one. It touches only `slug`,
+    /// so `name`, `export_epoch` and the lock/release timestamps are never disturbed by a late
+    /// arrival.
     pub async fn create(pool: &PgPool, slug: &str, name: &str) -> Result<Self, sqlx::Error> {
-        sqlx::query_as::<_, Self>("INSERT INTO event (slug, name) VALUES ($1, $2) RETURNING *")
+        sqlx::query_as::<_, Self>(
+            "INSERT INTO event (slug, name) VALUES ($1, $2)
+             ON CONFLICT (slug) DO UPDATE SET slug = EXCLUDED.slug
+             RETURNING *",
+        )
         .bind(slug)
         .bind(name)
         .fetch_one(pool)
         .await
     }
 
+    /// Reads first so the common case (the row already exists, i.e. every join after the first)
+    /// stays a plain SELECT and never takes a row lock.
     pub async fn find_or_create(
         pool: &PgPool,
         slug: &str,
@@ -51,3 +69,71 @@ impl Event {
         Self::create(pool, slug, name).await
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// The QR code goes up and every phone posts `/join` in the same second, before the event row
+    /// exists. `find_or_create` reads first, so all of them miss, and all of them insert.
+    ///
+    /// With a bare `INSERT`, exactly one wins and the rest get a unique violation on `slug` —
+    /// surfaced as a 500 on the first screen of the event, for everyone but the winner. There is no
+    /// retry on that path and nothing in the UI explains it.
+    #[sqlx::test]
+    async fn concurrent_first_joins_all_get_the_same_event(pool: PgPool) {
+        let racers: Vec<_> = (0..16)
+            .map(|_| {
+                let pool = pool.clone();
+                tokio::spawn(
+                    async move { Event::find_or_create(&pool, "wedding", "Hochzeit").await },
+                )
+            })
+            .collect();
+
+        let mut ids = Vec::new();
+        for r in racers {
+            let event = r
+                .await
+                .expect("task panicked")
+                .expect("a concurrent first join must not fail — this is the QR-scan burst");
+            ids.push(event.id);
+        }
+
+        assert_eq!(ids.len(), 16);
+        assert!(
+            ids.iter().all(|id| *id == ids[0]),
+            "every racer must land on ONE event row, not create rivals"
+        );
+
+        let count: i64 = sqlx::query_scalar("SELECT COUNT(*) FROM event WHERE slug = 'wedding'")
+            .fetch_one(&pool)
+            .await
+            .expect("count");
+        assert_eq!(count, 1, "exactly one event row may exist for a slug");
+    }
+
+    /// A late arrival must not clobber the row it collides with — the no-op `DO UPDATE` exists to
+    /// return the loser a row, not to let it rewrite one mid-event.
+    #[sqlx::test]
+    async fn a_late_create_does_not_disturb_the_existing_row(pool: PgPool) {
+        let first = Event::find_or_create(&pool, "wedding", "Hochzeit")
+            .await
+            .expect("first");
+
+        sqlx::query("UPDATE event SET name = $1, export_epoch = 7 WHERE id = $2")
+            .bind("Anna und Ben")
+            .bind(first.id)
+            .execute(&pool)
+            .await
+            .expect("simulate a live event");
+
+        let late = Event::create(&pool, "wedding", "Hochzeit")
+            .await
+            .expect("a colliding insert must still return the row");
+
+        assert_eq!(late.id, first.id);
+        assert_eq!(late.name, "Anna und Ben", "the name must survive");
+        assert_eq!(late.export_epoch, 7, "and so must the export epoch");
+    }
+}