feat(obs): add provider inventory and capability coverage

[typelicious]

What changed

• add top-level capability coverage and provider summary to GET /health
• add GET /api/providers for filtered provider inventory and dashboard metadata
• extend the no-build dashboard with capability coverage and richer provider capability visibility
• expose provider image metadata through the operational inventory surface
• update README, architecture, roadmap, changelog, and tests

Why

Modality-aware routing was already in the runtime, but operators still lacked a compact view of which loaded providers actually cover each capability and whether that coverage is currently healthy.

How verified

• PYTHONPYCACHEPREFIX="$PWD/.pycache" python3 -m compileall foundrygate tests
• PYTHONPATH=. ./.venv-check-313/bin/pytest -q
• ./.venv-check-313/bin/ruff check .
• ./.venv-check-313/bin/ruff format --check .
• /usr/bin/git diff --check