perf: index usage rollups

[felixvippp-ai]

Closes #59

Summary

• Replaced full usageStore scans on metrics, stats, agent rollups, service rollups, top agents, and protocol billing totals with maintained usage indexes.
• Added an indexed usageStore wrapper that keeps byAgent, byService, per-agent totals, per-service totals, and protocol-wide outstanding request totals in sync on set, delete, and clear.
• Preserved existing response shapes and key convention ${agent}::${serviceId}.
• Added assertUsageIndexesConsistent() for test-only brute-force parity checks.
• Added focused regression tests covering add, bulk usage, settle-to-zero, direct overwrite, clear, and indexed agent/service route outputs.

Validation

• npm.cmd run build
• $env:NODE_ENV='test'; node --test dist/*.test.js dist/**/*.test.js
• npm.cmd run lint
• npx.cmd prettier --check src/store/state.ts src/routes/usage.ts src/routes/services.ts src/routes/metrics.ts src/usage-index.test.ts
• git diff --check

Full test result from the Windows-compatible equivalent of npm test:

tests 71
suites 6
pass 71
fail 0
cancelled 0
skipped 0
todo 0

Note: the repository npm test script uses POSIX NODE_ENV=test syntax, which Windows cmd.exe cannot execute directly. I ran the same built test set with PowerShell's $env:NODE_ENV='test' before invoking node --test.

Complexity Note

Before this change, hot read paths repeatedly scanned every usage key and split string keys per request. After this change:

• /api/v1/metrics and /api/v1/stats read maintained totals directly.
• /api/v1/agents, /api/v1/agents/:agent/total, and /api/v1/agents/:agent/usage use agent indexes.
• /api/v1/services/:serviceId/usage, /api/v1/services/:serviceId/agents, and /api/v1/services/:serviceId/agents/top use service indexes.
• /api/v1/billing/total iterates per-service totals instead of every agent/service pair.

[mikewheeleer]

thanks for the effort @felixvippp-ai 🙏 this issue isn't assigned to you — please claim an open unassigned issue first, then open your PR. closing for now, hope to see it back 🙌