Revisit fiberflatnight requirements

The current pipeline will happily submit a fiberflat job for e.g. 30 cameras, even if it only has psfnight inputs for fewer cameras. Historically we have been very strict about how recent psf's need to be in order to process flats effectively. We should revisit this policy and either update recommendations or update the code to be equally strict.

Start by tracing fiberflat job submission and its psfnight input requirements; no files or tests are named. Clarify with maintainers the required camera coverage and PSF recency, and whether recommendations or enforcement should change. Done means the agreed policy is reflected in documentation or enforced by the pipeline, with validation for missing camera inputs.

Written by the indexing model from the issue text.