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

Uses gpuhunt==0.1.28, the first release containing the Seeweb provider from dstackai/gpuhunt#246.

Testing

• uv run --no-sync pytest src/tests/_internal/core/backends/seeweb src/tests/_internal/server/routers/test_backends.py — 67 passed, 36 skipped
• uv run --no-sync ruff check . — passed
• uv run --no-sync pyright -p . — 0 errors
• gpuhunt.providers.seeweb.SeewebProvider import with gpuhunt 0.1.28 — passed

AI Assistance

This PR was prepared and validated with OpenAI Codex assistance.

[massaindustries]

@r4victor This is ready for review. The dependency is now pinned to the released gpuhunt==0.1.28; the Seeweb/backend-router tests pass (67 passed, 36 skipped), and Ruff and Pyright are clean.