fix(ansible): ai-stack role uses python3.12 for venv + fix pip cache (#3534)

[mrveiss]

Summary

• Root cause: ai-stack Ansible role created the venv with python3 (resolves to Python 3.10 on target hosts), but requirements-ai.txt requires numpy>=2.4.3 which only exists for Python 3.11+. The backend role correctly uses python3.12; the ai-stack role did not.
• Stale venv guard: Added version check before creation so a pre-existing Python 3.10 venv is removed rather than silently reused (the creates: guard would otherwise skip re-creation).
• Pip cache: autobot-ai service account has create_home: false so ~/.cache/pip never exists. Added pip cache dir under ai_data_dir and set HOME env var on all pip tasks to suppress the warning and enable caching. Closes bug(ansible): autobot-ai pip cache dir not owned by autobot-ai — cache disabled on every AI stack install #3535.

Changes

autobot-slm-backend/ansible/roles/ai-stack/tasks/main.yml:

• Add "Check existing venv Python version" — failed_when: false so it's a no-op on first deploy
• Add "Remove stale venv if not Python 3.12" — removes 3.10 venv before re-creation
• python3 -m venv → python3.12 -m venv
• Add "Create pip cache directory for ai user" under ai_data_dir/.cache/pip
• Add HOME: "{{ ai_data_dir }}" to all three pip tasks (upgrade pip, install from requirements, fallback install)

Test plan

• Re-run fleet provisioning against a node with ai-stack role — Phase 5a should succeed
• Verify venv is at .../venv/lib/python3.12/site-packages on target host
• Verify no pip cache warning in provisioning output
• Re-run against a host with a pre-existing Python 3.10 venv — stale venv is detected and removed

Closes #3534
Closes #3535

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[github-actions]

✅ SSOT Configuration Compliance: Passing

🎉 No hardcoded values detected that have SSOT config equivalents!

[mrveiss]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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