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/backend/src/db/schema/stores.ts

@@ -38,7 +38,16 @@ export const locations = pgTable('location', {
   updatedAt: timestamp('updated_at', { withTimezone: true }).notNull().defaultNow(),
 })
 
+export const appConfig = pgTable('app_config', {
+  key: varchar('key', { length: 100 }).primaryKey(),
+  value: text('value'),
+  description: text('description'),
+  updatedAt: timestamp('updated_at', { withTimezone: true }).notNull().defaultNow(),
+})
+
 export type Company = typeof companies.$inferSelect
 export type CompanyInsert = typeof companies.$inferInsert
 export type Location = typeof locations.$inferSelect
 export type LocationInsert = typeof locations.$inferInsert
+export type AppConfig = typeof appConfig.$inferSelect
+export type AppConfigInsert = typeof appConfig.$inferInsert