fix(economy): BEGIN IMMEDIATE for write transactions (concurrency-safe)

Root cause of the fresh-DB multi-connection write failure: economy writes ran in
a DEFERRED transaction (pool.begin() = BEGIN) that read then wrote; SQLite returns
SQLITE_BUSY (code 5, 'database is locked') *immediately* when a deferred tx
upgrades to a write while another holds the write lock, bypassing busy_timeout to
avoid deadlock. Fix: open each write op with BEGIN IMMEDIATE on a dedicated pooled
connection (finish() commits/rolls back), taking the write lock up front so
busy_timeout serializes writers. Reproduction test: 100 fresh DBs x 8 concurrent
grant_reward — was 644/800 failures, now 800/800 succeed with correct final
balance (no lost update). Reads unchanged.

tests/concurrency_repro.rs

@@ -0,0 +1,65 @@
+//! Reproduction for the fresh-DB multi-connection warm-up write failure.
+//! Forces several pooled connections to open concurrently on a brand-new DB and
+//! captures the ACTUAL sqlx/SQLite error (not the service's generic string).
+
+use openfut_core::db::{init_pool, run_migrations};
+use openfut_core::services::economy;
+
+async fn seed_club(pool: &sqlx::SqlitePool) {
+    sqlx::query(
+        "INSERT INTO profiles (id, username, created_at, updated_at) VALUES ('p','p','t','t')",
+    )
+    .execute(pool)
+    .await
+    .unwrap();
+    sqlx::query("INSERT INTO clubs (id, profile_id, name, coins, created_at, updated_at) VALUES ('c','p','c',100000,'t','t')")
+        .execute(pool)
+        .await
+        .unwrap();
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 8)]
+async fn fresh_db_multiconn_concurrent_writes() {
+    let base = std::env::temp_dir().join(format!("ofut-cc-{}", std::process::id()));
+    std::fs::create_dir_all(&base).unwrap();
+    let iters = 100usize;
+    let mut failures = 0usize;
+    let mut first_err = String::new();
+    for i in 0..iters {
+        let url = format!("sqlite://{}/db{i}.db", base.display());
+        let pool = init_pool(&url, 5).await.expect("init_pool");
+        run_migrations(&pool).await.expect("migrations");
+        seed_club(&pool).await;
+        // Fire concurrent credits to force several connections to warm up at once
+        // on the brand-new DB, then a write — the harness's failing shape.
+        let mut handles = Vec::new();
+        for _ in 0..8 {
+            let p = pool.clone();
+            handles.push(tokio::spawn(async move {
+                economy::grant_reward(&p, "c", 1).await
+            }));
+        }
+        for h in handles {
+            match h.await.unwrap() {
+                Ok(_) => {}
+                Err(e) => {
+                    failures += 1;
+                    if first_err.is_empty() {
+                        first_err = format!("{e:?}");
+                    }
+                }
+            }
+        }
+        // Serialization correctness: 8 concurrent +1 credits, no lost update.
+        let bal = economy::balance(&pool, "c").await.unwrap();
+        assert_eq!(bal, 100_008, "iter {i}: lost update under concurrency");
+        pool.close().await;
+    }
+    std::fs::remove_dir_all(&base).ok();
+    assert_eq!(
+        failures,
+        0,
+        "{failures}/{} iterations had a write failure; first error: {first_err}",
+        iters * 8
+    );
+}