CI green on 3.11/3.12: Mac-only MLX; fix E402 import order; lint & requirements tidy

[asq-sheriff]

Summary

Bring CI to green on Python 3.11 and 3.12 by fixing dependency and lint issues.

Changes

• Mark MLX packages as macOS/arm64-only via PEP-508 markers:
  • mlx==0.28.0; mlx-lm==0.26.3; mlx-metal==0.28.0
• Fix E402 (module imports at top) in app/utils/seed_data.py
• Remove unused variable from gc.collect() loop; drop extraneous f-string
• Minor requirements tidy

Why

• Ubuntu runners cannot install MLX packages; they are Mac-only (Apple Silicon)
• Ruff errors (E402 + small nits) were blocking the 3.12 job

Testing

• Local: ruff check . --fix and pytest -q (informational)
• CI: this PR should run the ci workflow (matrix 3.11/3.12) and pass

Checklist

• Lint passes (ruff)
• CI workflow runs on PR
• No secrets or env files included