fix: size the DB pool and fail fast on saturation

The single Postgres pool (shared by HTTP handlers and the crawler/analysis
daemons) hardcoded max_connections=10 with no acquire_timeout, so under
saturation callers hung on the driver's silent 30s default. Add a DbConfig
(DB_MAX_CONNECTIONS default 20, DB_ACQUIRE_TIMEOUT_SECS default 10, clamped
to >=1) and apply both in PgPoolOptions.

Bump to 0.124.4.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/app.rs

@@ -279,7 +279,8 @@ impl DaemonReloader for Supervisors {
 
 pub async fn build(config: Config) -> anyhow::Result<AppHandle> {
     let db = PgPoolOptions::new()
-        .max_connections(10)
+        .max_connections(config.db.max_connections)
+        .acquire_timeout(config.db.acquire_timeout)
         .connect(&config.database_url)
         .await?;
     sqlx::migrate!("./migrations").run(&db).await?;