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/services/config.service.ts

@@ -0,0 +1,46 @@
+import { eq } from 'drizzle-orm'
+import type { PostgresJsDatabase } from 'drizzle-orm/postgres-js'
+import { appConfig } from '../db/schema/stores.js'
+
+let configCache: Map<string, string | null> | null = null
+
+export const AppConfigService = {
+  async getAll(db: PostgresJsDatabase<any>) {
+    return db.select().from(appConfig)
+  },
+
+  async get(db: PostgresJsDatabase<any>, key: string): Promise<string | null> {
+    if (!configCache) await this.refreshCache(db)
+    return configCache!.get(key) ?? null
+  },
+
+  async set(db: PostgresJsDatabase<any>, key: string, value: string | null, description?: string) {
+    const [existing] = await db.select().from(appConfig).where(eq(appConfig.key, key)).limit(1)
+
+    if (existing) {
+      const [updated] = await db
+        .update(appConfig)
+        .set({ value, updatedAt: new Date(), ...(description !== undefined ? { description } : {}) })
+        .where(eq(appConfig.key, key))
+        .returning()
+      configCache = null
+      return updated
+    }
+
+    const [inserted] = await db
+      .insert(appConfig)
+      .values({ key, value, description, updatedAt: new Date() })
+      .returning()
+    configCache = null
+    return inserted
+  },
+
+  async refreshCache(db: PostgresJsDatabase<any>) {
+    const rows = await db.select({ key: appConfig.key, value: appConfig.value }).from(appConfig)
+    configCache = new Map(rows.map((r) => [r.key, r.value]))
+  },
+
+  invalidateCache() {
+    configCache = null
+  },
+}