chore: migrate etl_target → pts_target (rename pts_target → pts_pre_target)

[d0choa]

Summary

• Rename pts_target: step definition → pts_pre_target: in unified_pipeline.yaml (preprocessing step)
• Remove etl_target: block from unified_pipeline.yaml
• Add pts_target: block with correct dependencies (pts_pre_target, pts_reactome, pts_target_safety, pts_target_gene_essentiality, pts_disease)
• Replace all 26 occurrences of - etl_target with - pts_target in unified_pipeline.yaml
• Update gentropy_study and pts_drug_mechanism_of_action depends_on to reference pts_pre_target
• Rename target: → pre_target: in pts.yaml and add new target: pyspark step
• Remove target { ... } block from etl.conf, leaving a stub with output path

Validation

grep "etl_target" src/orchestration/dags/config/unified_pipeline.yaml
# Expected: empty output ✓

Test plan

• No etl_target references remain in unified_pipeline.yaml
• DAG validation in staging environment

Tracking issue: opentargets/issues#4347