Handle Job.DoesNotExist exception properly in process_source_images_async

[coderabbitai]

Problem Description

In ami/ml/tasks.py, the process_source_images_async function catches Job.DoesNotExist exceptions but doesn't reset job_id to None. This means the invalid job_id is passed to downstream functions (process_images() and save_results()), which could cause errors.

Additionally, the exception handler uses logger.error() instead of logger.exception(), which means the full traceback is not captured for debugging.

Context

• File: ami/ml/tasks.py
• Function: process_source_images_async
• Related PR: feat: Project feature flag for reprocessing all images #1053
• Review Comment: feat: Project feature flag for reprocessing all images #1053 (comment)

Expected Behavior

When a Job.DoesNotExist exception occurs:

1. The job_id variable should be set to None to prevent passing an invalid ID downstream
2. The full exception traceback should be logged using logger.exception() for debugging

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Requested by: @mihow