Restores the Phase 0 security features that were lost during the merge of
SyslogSolution/syslog-harness into abiba-bot/inference-harness (commit 815ed799).
Changes:
- Added ADMIN_KEY env var + _admin_auth() for endpoint protection
- Added GET /admin/keys — list all keys with deprecation status
- Added GET /admin/keys/deprecation-summary — deprecated key usage from Redis
- Added POST /admin/keys/generate — generate new keys (memory-only)
- Added POST /admin/keys/revoke — revoke deprecated keys (memory-only)
- Added dual-key deprecated key logging to chat() endpoint
- Preserves: Redis lazy reconnect, MoE spillover, 5-tier routing, vision guard
