Cache leaderboard endpoints with lifecycle invalidation

[silentgeckoaudit3801]

Closes #238.

Summary

• cache /api/get-user-leaderboard and /api/get-position-tokens-statistic behind short-lived Redis keys
• add best-effort synchronous leaderboard cache invalidation for position lifecycle hooks
• invalidate leaderboard aggregates after position open, close, and liquidation updates
• add static coverage for endpoint cache wiring and lifecycle invalidation wiring

Validation

• Parsed the changed Python files with ast.parse using the bundled Codex Python runtime.
• GitHub compare reports this branch is 0 commits behind main.

Testing

• Not run as a project test suite because this workspace is avoiding dependency installation and project toolchain execution.