feat: per-target deployment queues with configurable concurrency

[BSalaeddin]

Adds per-server deployment queues with configurable concurrency and self-hosted deployment cancellation.

What changed

• Each server gets its own BullMQ queue instead of a shared global one
• Concurrency is configurable per server (1–10) from the UI
• Deployment cancellation now works on self-hosted (previously cloud-only)
• Existing queue name preserved so upgrades don't strand pending jobs

Migration

Two new deploymentConcurrency columns (server + webServerSettings), default 1 — no behavior change on upgrade.

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