Support standalone deployment server mode

[holmofy]

Title: Feature Request: Provide an apalis-server for standalone deployment (Faktory-like mode)

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Description
First of all, thanks for building Apalis — it’s a great library for background job processing in Rust.

Currently, Apalis works very well as an embedded library, but in some use cases it would be very helpful to have a standalone server mode, similar to how [Faktory](https://github.com/contribsys/faktory) works.

With a standalone apalis-server, developers could:

• Deploy a single job processing service in their infrastructure.
• Connect to it from multiple applications (potentially across different languages).
• Manage and monitor jobs from a central place.

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Motivation

• In multi-service environments, having each service embed Apalis separately makes management harder.
• A central server improves observability, monitoring, and operational simplicity.
• It also allows the project to define a simple protocol (HTTP/gRPC/custom TCP), which opens the door for community clients in other languages (e.g. Go, Python, JavaScript).

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Proposed solution

• Introduce a new crate/binary, e.g. apalis-server.

• This binary would:

  • Run the Apalis job engine internally.
  • Expose a simple API/protocol for pushing/pulling jobs.
  • Optionally provide a basic Web UI and metrics endpoint.

• Keep Apalis as a library for applications that prefer direct integration.

• This way, Apalis can cover both use cases: embedded mode and server mode.

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Inspiration
The model of Faktory is a good reference:

• Simple client protocol.
• Reliable job storage (via Redis or Postgres).
• Features like retries, scheduled/delayed jobs, and failure queues.
• A Web UI for visibility (even a basic one would be very useful).

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Additional context
This would make Apalis not just a Rust job processing library, but a general-purpose job queue system that could be adopted across language ecosystems.