fix(queue): lease messages with FOR UPDATE SKIP LOCKED so two readers never share one

Queue.read() selected the visible candidates and then bumped their vt in a
second statement. On libSQL the single multiplexed connection and the
in-process write lock serialised readers, so that was safe; on the Postgres
pool (#239) two concurrent readers see the same candidates and both lease
them. The integration test 'never hands the same message to two readers'
failed against a real Postgres (36 leases of 10 messages). The select now
locks the rows it is about to lease and skips rows another reader holds.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: ralyodio

packages/shared/lib/queue.js

@@ -78,7 +78,8 @@ export class Queue {
            from queue_messages
           where queue_name = ? and vt <= ?
           order by msg_id
-          limit ?`,
+          limit ?
+            for update skip locked`,
         [this.name, nowIso, qty],
       );