feature: centralizes Celery task handling in shared helpers and configures result expiration

[JoeGaffney]

No description provided.

[Copilot]

Pull request overview

This PR centralizes Celery task handling in shared helpers and configures result expiration, while simplifying the API routers to use the new helpers and common settings.

Changes:

• Add a result_expires_days setting in both API and worker configs and wire it into the Celery app configuration to control task result TTL.
• Introduce create_task and get_task_detailed helpers in api/common/task_helpers.py, and refactor the images, videos, texts, and workflows routers to use them instead of direct celery_app access.
• Improve task introspection by combining queue-position lookup and Flower metadata, and change the behavior for unknown/expired tasks to return a 404 instead of a "task not found" log with 200.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
workers/common/config.py	Adds result_expires_days setting for worker-side Celery result expiry.
workers/worker.py	Configures worker Celery app to expire results based on settings.result_expires_days.
api/common/config.py	Adds API-side result_expires_days setting to keep API and worker Celery configs aligned.
api/worker.py	Configures API Celery client to use result_expires_days for result expiry.
api/common/task_helpers.py	Refactors task utilities to include a unified create_task, a richer get_task_detailed (with Flower and Redis queue info plus 404 on missing tasks), and a simplified cancel_task.
api/images/router.py	Refactors image endpoints to use create_task/get_task_detailed and the new cancel_task signature, while preserving response models and behavior.
api/videos/router.py	Same refactor pattern as images router for video task creation, polling, and cancellation.
api/texts/router.py	Refactors text endpoints to use the shared helpers, simplifying task creation and polling.
api/workflows/router.py	Refactors workflow endpoints to use the shared helpers and maintain signed URL generation for outputs.