perf: admin log collector runs on the hot path even when the admin API is disabled

[skyoo2003]

Context

Surfaced during code review of #111 (finding 4, skipped as pre-existing).

Problem

cmd/devcloud/main.go always builds a 1000-entry admin.LogCollector, and the gateway's logging middleware (internal/gateway/gateway.go, in New) calls logCollector.Add(...) — a mutex lock + ring-buffer insert — on every AWS request, regardless of admin.enabled.

When admin.enabled: false (the default), nothing can ever read those logs without a restart, so every request pays for bookkeeping no consumer can reach.

Why it was deferred

Pre-existing behavior (not introduced by #111). Conditionalizing it requires a gateway.New signature/behavior change (e.g. inject an optional collector or a no-op), which is out of proportion to that PR's diff.

Suggested fix

Make the log collector optional: when admin is disabled, skip the Add call (nil collector / no-op recorder) so the middleware short-circuits on the hot path. Keep behavior identical when admin is enabled.

Acceptance

• No logCollector.Add work per request when admin.enabled: false.
• Admin-enabled behavior (recent-logs endpoint) unchanged, still covered by tests.