feat(obs): add client usage reporting

[typelicious]

What changed

• add richer client usage aggregates to the metrics store
• expose aggregate client totals through GET /api/stats
• extend the dashboard with top-client, client totals, token, failure, success-rate, and cost-per-request views
• update metrics tests and API docs

Why

• the next v1.1 operations slice should make client traffic visible, not just total traffic
• token and cost visibility per client is already stored; this slice surfaces it where operators can use it
• this sets up the later routing-rule review with better evidence

How verified

• python3 -m compileall foundrygate tests
• PYTHONPATH=. ./.venv-check-313/bin/pytest -q tests/test_metrics_traces.py
• ./.venv-check-313/bin/ruff check foundrygate/metrics.py foundrygate/main.py tests/test_metrics_traces.py
• /usr/bin/git diff --check