Api: Add background task queue

[losolio]

Plumbing a new IBackgroundTaskQueue. No consumers wired up in this slice; the $archive-import PR will be the first user.

[Copilot]

Pull request overview

Adds the initial API-side plumbing for an in-process background task queue that can run queued work items through a hosted service with per-item DI scopes.

Changes:

• Introduces IBackgroundTaskQueue and a Channel-backed implementation.
• Adds QueuedHostedService to drain queued work items and isolate each item in a DI scope.
• Registers the queue/hosted service in Program.cs and adds unit tests for execution, scoped resolution, and failure isolation.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Ignis.Api/Program.cs	Registers the background task queue and hosted drainer.
src/Ignis.Api/Services/BackgroundTasks/IBackgroundTaskQueue.cs	Defines the queue contract for producers and the hosted consumer.
src/Ignis.Api/Services/BackgroundTasks/BackgroundTaskQueue.cs	Implements the queue with an unbounded single-reader channel.
src/Ignis.Api/Services/BackgroundTasks/QueuedHostedService.cs	Runs queued delegates one at a time inside scoped service providers.
tests/Ignis.Api.Tests/BackgroundTaskQueueTests.cs	Adds coverage for queued execution, scoped service resolution, and resilience after failures.

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