feat(studio): add agent usage statistics

[evanlowe]

Summary

• record successful Studio Agent invocations as immutable TOS events using trusted server-side user identity
• expose an authorized, paginated usage summary API with invocation totals and per-user activity
• add a 用量统计 tab to Agent details with loading, empty, retry, and pagination states
• document the storage layout and refresh the bundled Studio assets

Validation

• npm test — 466 passed
• npm run build — passed
• Agent usage/storage tests — 19 passed
• runtime proxy and Studio RBAC regression tests — 81 passed
• pre-commit (ruff check, ruff format, hardcoded secret detection) — passed
• Pyright on the new backend and tests — 0 errors
• verified a live TOS event aggregates to 1 invocation, 1 user, and 1 detail row

Notes

• writes are intentionally best-effort so TOS failures never interrupt Agent responses
• the initial implementation aggregates immutable events at read time; compaction/indexing can be added if event volume requires it