[queue] Pluggable queue drivers (memory / unstorage / qstash / sqs)

[productdevbook]

Part of #24.

```ts
import { createEmail } from 'unemail'
import memoryQueue from 'unemail/queue/memory'
import qstashQueue from 'unemail/queue/qstash'

const email = createEmail({
driver: resend(...),
queue: memoryQueue({ concurrency: 5 }),
// or: queue: qstashQueue({ token, url })
})

await email.enqueue({ to, subject, react: , scheduledAt: '3d' })
```

• Queue interface: `enqueue`, `process`, `fail`, `retry`
• In-memory + unstorage adapter (persists to any unstorage driver)
• QStash adapter (serverless-friendly)
• SQS adapter

[productdevbook]

Delivered in PR #56 (commit 1843406).

unemail/queue ships four pieces:

• EmailQueue interface — minimal contract (enqueue / pull / ack / fail / size) so backends plug in without changing producers.
• memoryQueue() — single-process in-memory implementation with injectable clock for tests.
• unstorageQueue({ storage }) — durable queue backed by any unstorage driver. UnstorageLike interface is structurally typed so no peer install is required for core users.
• startWorker(email, queue, opts) — concurrency + maxAttempts + pluggable exponential backoff, plus start() / stop() / waitForIdle() / tick() (manual tick for tests).

Exports: unemail/queue, unemail/queue/memory, unemail/queue/unstorage, unemail/queue/worker.

QStash + SQS adapters remain easy follow-ups; they're single files that implement the 5-method contract. Left as future issues because their idiomatic surface (push vs pull) is better served by bespoke workers, not the generic loop here.