Reorganiza internal/ en capas: domain (entities, repositories),
usecase (run_sync_pipeline, scheduler) e infrastructure (config,
database/postgres, database/mongo, http). El cableado por pipeline
que vivia inline en main.go pasa a internal/infrastructure/pipeline,
de modo que agregar un pipeline ya no obliga a tocar main.

Agrega health handler y pkg/changelog; jsonutil se mueve a pkg/.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>