PSv2: add push-mode worker to the official processing service templates

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• Required to be added to the minimal template so that we can run Django tests
• A more complete example can be added to the "example" template in a follow up

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Claude says: Related in-flight work: PR #1252 (draft) — v2 worker mode for the minimal stub — adds the pull/async worker to processing_services/minimal/ so CI can exercise the NATS/Redis/Celery lifecycle without the external ADC worker. That is the direct implementation of this issue; tracking it there. (Also referenced by the PSv2-default rollout #1353.)