fix(projects): honor custom slug in organization project creation

[fallintoplace]

Summary

• pass the validated project slug through the organization-level creation endpoint
• preserve automatic slug generation when no slug is supplied
• cover custom slugs in both the API response and persisted project

Why

The organization-level endpoint validates an optional slug but previously discarded it during project creation. This made its behavior inconsistent with the team-level endpoint and silently replaced caller-supplied slugs with generated ones.

Testing

• repository Python pre-commit checks
• mypy pre-push check
• focused ruff check

[cursor]

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