db: enable WAL and index the pending-task lease query

The lease/earliest_run_after queries full-scanned tasks, and the
rollback journal blocked readers behind worker writes. journal_mode=WAL
(persistent, idempotent) plus idx_tasks_pending(status, run_after)
covers both without a schema migration.

crates/xmedia-bot/src/db.rs

@@ -16,6 +16,10 @@ use std::time::Duration;
 pub fn open_db(path: &str) -> rusqlite::Result<Connection> {
     let conn = Connection::open(path)?;
     conn.busy_timeout(Duration::from_secs(5))?;
+    // WAL lets readers run alongside writer leases instead of blocking on
+    // the rollback journal; the mode persists in the DB header, so the
+    // idempotent pragma here and in ensure_schema only needs to win once.
+    conn.pragma_update(None, "journal_mode", "WAL")?;
     Ok(conn)
 }
 

crates/xmedia-bot/src/queue.rs

@@ -83,9 +83,11 @@ fn scaled_retry_delay(base: f64, attempts: i32) -> f64 {
 
 fn ensure_schema(conn: &rusqlite::Connection) -> rusqlite::Result<()> {
     conn.execute_batch(
-        "CREATE TABLE IF NOT EXISTS tasks (id TEXT PRIMARY KEY, payload TEXT NOT NULL, \
+        "PRAGMA journal_mode=WAL; \
+         CREATE TABLE IF NOT EXISTS tasks (id TEXT PRIMARY KEY, payload TEXT NOT NULL, \
          run_after REAL NOT NULL, attempts INTEGER NOT NULL, status TEXT NOT NULL, \
-         locked_until REAL NOT NULL, created_at REAL NOT NULL);",
+         locked_until REAL NOT NULL, created_at REAL NOT NULL); \
+         CREATE INDEX IF NOT EXISTS idx_tasks_pending ON tasks(status, run_after);",
     )
 }