debug/admin routes

[yamcodes]

Overview

Add a set of debug/admin endpoints to the engine for observability and manual control.

Potential routes

• GET /admin/sessions — list all active sessions (session ID, last active, game state)
• GET /admin/sessions/{session_id} — inspect a specific session
• DELETE /admin/sessions/{session_id} — manually evict a session
• GET /admin/stats — memory/session count summary

Notes

• Useful for debugging the session memory leak (see session lifecycle management: eviction, resource cleanup, and thread safety #10)
• Keep separate from the public API (prefix /admin/)
• Auth can be revisited when deployment is a concern