- script_template.py: add server_default to ALL NOT NULL columns so
Base.metadata.create_all matches Alembic behavior for raw SQL INSERTs
- test_session_branches_api.py: fork_reason needs 5+ chars ("test" → "testing fork")
- test_scripts.py: engineers CAN create templates (assert 201, not 403)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>