Add Seeweb GPU backend

[massaindustries]

Summary

• Add Seeweb as a configurable dstack GPU backend.
• Query authenticated offers and capacity, provision and terminate servers, and manage SSH keys.
• Handle image selection, provisioning status, capacity errors, GPU setup, and the Seeweb first-boot reboot.
• Add backend documentation, frontend typing, and unit coverage.

Dependency

Depends on dstackai/gpuhunt#246 and a subsequent gpuhunt release containing the Seeweb provider.
The pinned gpuhunt version in dstack must be updated after that release.

Testing

Validated with the local gpuhunt Seeweb provider:

• uv run --no-sync pytest -n auto src/tests --runui --runpostgres — 4,473 passed, 4 skipped
• Targeted Seeweb and backend registration tests — 31 passed
• Ruff and pre-commit checks passed
• Gitleaks commit scan — no leaks found

AI Assistance

This PR was prepared and validated with OpenAI Codex assistance.