fix: handle nested image count errors safely

[Minorli]

Summary

• recognize nested upstream candidate-count parameters such as tools[0].n
• fall back without n only for explicit HTTP 400 parameter rejections
• avoid retries for ambiguous gateway failures and moderation messages
• release PicGen 0.1.60

Test plan

• 393 pytest tests passed
• 87% coverage (fail-under 80)
• Ruff, Mypy, Node syntax, Shell syntax, Compose config, uv lock, diff check passed
• pip-audit reported no known dependency vulnerabilities
• independent review: no remaining findings