perf: fix API CPU saturation at 400+ devices

Root cause: stale NATS JetStream consumers accumulated across API
restarts, causing 13+ consumers to fight over messages in a single
Python async event loop (100% CPU).

Fixes:
- Add performance indexes on devices(tenant_id, hostname),
  devices(tenant_id, status), key_access_log(tenant_id, created_at)
  — drops devices seq_scans from 402k to 6 per interval
- Remove redundant ORDER BY t.name from fleet summary SQL
  (tenant name sort is client-side, was forcing a cross-table sort)
- Bump NATS memory limit from 128MB to 256MB (was at 118/128)
- Increase dev poll interval from 60s to 120s for 400+ device fleet

The stream purge + restart brought API CPU from 100% to 0.3%.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.override.yml

@@ -80,7 +80,7 @@ services:
       CREDENTIAL_ENCRYPTION_KEY: ${CREDENTIAL_ENCRYPTION_KEY:?Set CREDENTIAL_ENCRYPTION_KEY in .env}
       OPENBAO_ADDR: http://openbao:8200
       OPENBAO_TOKEN: dev-openbao-token
-      POLL_INTERVAL_SECONDS: 60
+      POLL_INTERVAL_SECONDS: 120
       WIREGUARD_GATEWAY: wireguard
       TUNNEL_PORT_MIN: 49000
       TUNNEL_PORT_MAX: 49004