billing: expose deposit capacity preflight

[halfaipg]

Summary

• expose today’s credited account and network usage in funding configuration
• calculate non-negative remaining capacity for each funding asset
• preserve locked server-side cap enforcement as the authority

Verification

• PYTHONPATH=. .venv/bin/pytest -q grid_api (374 passed, 8 skipped)
• focused deposit tests (21 passed)
• fatal ruff checks and git diff --check