Add et_db_diff_sourceimages() and /sourceimages/diff endpoint

Lars Dalbyrequested to merge
dev into main

Diff sourceimages against the DB to skip already-inserted rows. Integrates into /sourceimages/write (reports rows_new) and adds a synchronous /sourceimages/diff preview endpoint. Both endpoints support deployment lookup via (projectid, partnerid, year, trap_code). Also adds batch_size validation to both write and diff functions.

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com

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