feat: add app_config table with runtime log level control and POS structured logging

- New app_config key-value table for system settings, with in-memory cache (mirrors ModuleService pattern)
- GET/PATCH /v1/config endpoints for reading and updating config (settings.view/settings.edit permissions)
- Runtime log level: PATCH /v1/config/log_level applies immediately, persists across restarts
- Startup loads log level from DB in onReady hook (env var is default, DB overrides)
- Add structured request.log.info() to POS routes: transaction create/complete/void, drawer open/close, discount create/update/delete

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

packages/shared/src/schemas/index.ts

@@ -191,3 +191,6 @@ export type {
   DrawerOpenInput,
   DrawerCloseInput,
 } from './pos.schema.js'
+
+export { LogLevel, AppConfigUpdateSchema } from './config.schema.js'
+export type { AppConfigUpdateInput } from './config.schema.js'