sessions: expire after 1 day

Sessions previously had no expiry at all: a county_session cookie
was valid forever until an explicit /logout. Both session lookups
now reject rows older than 1 day (created_at-based, not sliding),
and db::delete_expired_sessions prunes expired rows so the table
doesn't grow unbounded; it's called during the existing OAuth-start
housekeeping alongside the oauth_states cleanup.

src/auth.rs

@@ -35,6 +35,10 @@ pub async fn start(State(state): State<AppState>) -> impl IntoResponse {
         return "internal error".into_response();
     }
 
+    if let Err(error) = db::delete_expired_sessions(&state.db).await {
+        tracing::error!(%error, "cannot clean expired sessions");
+    }
+
     if let Err(error) = sqlx::query("INSERT INTO oauth_states(state) VALUES ($1)")
         .bind(&oauth_state)
         .execute(&state.db)

src/db.rs

@@ -38,6 +38,7 @@ pub async fn athlete_for_session(db: &PgPool, token: &str) -> Result<Option<(i64
         FROM sessions s
         JOIN athletes a ON a.id = s.athlete_id
         WHERE s.token_hash = $1
+          AND s.created_at > now() - interval '1 day'
         "#,
     )
     .bind(&hash)
@@ -89,6 +90,7 @@ pub async fn session_athlete(db: &PgPool, token: &str) -> Result<Option<SessionA
         JOIN athletes a
           ON a.id = s.athlete_id
         WHERE s.token_hash = $1
+          AND s.created_at > now() - interval '1 day'
         "#,
     )
     .bind(&hash)
@@ -343,6 +345,23 @@ pub async fn mark_activity_processed(
     Ok(())
 }
 
+/// Remove sessions past their 1-day validity window.
+///
+/// Expired sessions are already rejected by `session_athlete` /
+/// `athlete_for_session`; this just keeps the table from growing forever.
+pub async fn delete_expired_sessions(db: &PgPool) -> Result<()> {
+    sqlx::query(
+        r#"
+        DELETE FROM sessions
+        WHERE created_at <= now() - interval '1 day'
+        "#,
+    )
+    .execute(db)
+    .await?;
+
+    Ok(())
+}
+
 pub async fn delete_session(db: &PgPool, token: &str) -> Result<()> {
     let hash = hash_token(token);