fix: allowlist opusContext1mEnabled in SettingsUpdateSchema (#78)

Same root cause as the thinkingEffort fix in #73: the schema is
.strict(), so unknown keys in PUT /api/settings are rejected with
INVALID_INPUT and the toggle never persists. Verified live:
pre-fix returned {"errorCode":"INVALID_INPUT"}, post-fix accepts.

The frontend has been reading and writing this key for a while
(settings-ui.js:336, :1137; session-ui.js:331), so saves were
silently failing — users never noticed because the load path
falls back to false on missing keys.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>