fix(db): serialize SQLite WAL establishment before pooling

Switching a brand-new DB file to WAL is a one-time file-level change; letting
multiple pooled connections perform it concurrently during warm-up races the
switch and can surface a spurious lock (observed as intermittent 500s under the
integration harness). Open ONE connection to establish WAL before the pool
opens, so every pooled connection thereafter only re-asserts an already-WAL
file. Keeps per-connection foreign_keys + busy_timeout.

src/db.rs

@@ -1,7 +1,7 @@
 use anyhow::Result;
 use sqlx::{
     sqlite::{SqliteConnectOptions, SqliteJournalMode, SqlitePoolOptions},
-    SqlitePool,
+    ConnectOptions, Connection, SqlitePool,
 };
 use std::str::FromStr;
 use std::time::Duration;
@@ -11,16 +11,26 @@ pub type Pool = SqlitePool;
 
 pub async fn init_pool(database_url: &str, max_connections: u32) -> Result<Pool> {
     info!("Connecting to database: {}", database_url);
-    // Per-connection options so EVERY pooled connection gets them (a one-off
-    // `PRAGMA` on the pool only configures whichever connection served it):
-    // WAL for reader/writer concurrency, foreign keys on, and a busy_timeout so
-    // a transient SQLITE_BUSY under concurrent access waits-and-retries instead
-    // of failing the transaction.
+    // Per-connection options so EVERY pooled connection gets them: WAL for
+    // reader/writer concurrency, foreign keys on, and a busy_timeout so a
+    // transient SQLITE_BUSY under concurrent access waits-and-retries.
     let opts = SqliteConnectOptions::from_str(database_url)?
         .create_if_missing(true)
         .journal_mode(SqliteJournalMode::Wal)
         .foreign_keys(true)
         .busy_timeout(Duration::from_secs(5));
+    // Establish WAL on the file via ONE connection BEFORE the pool opens.
+    // Switching a fresh DB to WAL is a one-time file-level change; letting
+    // several pooled connections do it concurrently at warm-up races that
+    // switch and can surface a spurious lock. Serialize it here so every
+    // pooled connection thereafter only re-asserts an already-WAL file.
+    {
+        let mut conn = opts.clone().connect().await?;
+        sqlx::query("PRAGMA journal_mode=WAL")
+            .execute(&mut conn)
+            .await?;
+        conn.close().await?;
+    }
     let pool = SqlitePoolOptions::new()
         .max_connections(max_connections)
         .connect_with(opts)