selfhost(queue): worker concurrency control (QUEUE_CONCURRENCY)

[JSONbored]

Part of #977.

Currently the SQLite and Postgres queues process one job at a time (single pump loop). Add a QUEUE_CONCURRENCY env var (default 1) so operators can saturate multi-core machines and the AI provider concurrently.

Scope:

• opts.concurrency param on createSqliteQueue and createPgQueue
• Falls back to QUEUE_CONCURRENCY env var (integer, ≥1)
• SQLite: multiple pump loops, each claiming via atomic status='pending' UPDATE
• Postgres: multiple async loops, each claiming via FOR UPDATE SKIP LOCKED
• stop() and drain() wait for active === 0
• Full branch coverage on new paths

[JSONbored]

Closing as completed (2026-07-01 issue audit): QUEUE_CONCURRENCY/opts.concurrency shipped in sqlite-queue.ts and pg-queue.ts (multi-loop, SKIP LOCKED).