perf(db): size the pool to cover every DB consumer

POOL_SIZE = 4 was the process-wide cap on concurrent DB operations while 4 queue workers, 8 URL workers, dispatcher handlers and the sweep all share the pool — WAL readers queued behind writers and every hot-path round trip (3-5 per message) contended for four permits. 16 covers every consumer at once; SQLite's single writer serializes writes regardless.

crates/xmedia-bot/src/db.rs

@@ -17,9 +17,11 @@ use std::sync::Arc;
 use std::time::Duration;
 
 /// Upper bound on pooled (reused) connections and on concurrent DB
-/// operations per store. Small on purpose: the queue's `BEGIN IMMEDIATE`
-/// leases serialize writes anyway, and WAL readers rarely need more.
-const POOL_SIZE: usize = 4;
+/// operations. Sized to cover every consumer at once — 4 queue workers +
+/// 8 URL workers, plus dispatcher handlers and the sweep — so the semaphore
+/// stops queueing operations behind each other; SQLite's single writer
+/// serializes writes regardless, and WAL readers rarely block.
+const POOL_SIZE: usize = 16;
 
 /// A tiny connection pool for one SQLite file. Connections are checked out
 /// on a blocking thread and returned afterwards; `acquire` opens a new