Add REST API (FastAPI) for run jobs, config, and status

[vecnode]

Wrap diffusion in a worker or queue; expose POST /runs, GET /runs/{id}. Same core as CLI, no logic duplicated in route handlers.

[vecnode]

Quick status + next steps for this REST API work:

What is in place already

• The core run logic is being refactored toward a modular pipeline and a single orchestrator entrypoint, which is a good fit for a worker/queue model.
• Configuration is being centralized, so the same Settings object can be reused by CLI, templates, and any API workers.

What remains to fully resolve this issue

• Implement a worker abstraction that can receive a Settings object + payload, execute a run, and report structured status/progress (start, running, completed, failed).
• Wrap that worker in a FastAPI app exposing at least:
  • POST /runs to enqueue a new run from JSON config (mapping directly onto the centralized Settings).
  • GET /runs/{id} to fetch status and outputs metadata.
• Ensure the FastAPI routes do not duplicate business logic: they should just validate input, construct Settings, and delegate to the worker/orchestrator.
• Add basic docs and an example client snippet so users can trigger runs remotely.

With the pipeline and config refactors, this API should be able to share almost all logic with the CLI/templates, making maintenance much easier.