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/main.ts

@@ -30,8 +30,10 @@ import { storeRoutes } from './routes/v1/store.js'
 import { vaultRoutes } from './routes/v1/vault.js'
 import { webdavRoutes } from './routes/webdav/index.js'
 import { moduleRoutes } from './routes/v1/modules.js'
+import { configRoutes } from './routes/v1/config.js'
 import { RbacService } from './services/rbac.service.js'
 import { ModuleService } from './services/module.service.js'
+import { AppConfigService } from './services/config.service.js'
 
 export async function buildApp() {
   const app = Fastify({
@@ -106,6 +108,7 @@ export async function buildApp() {
   await app.register(rbacRoutes, { prefix: '/v1' })
   await app.register(storeRoutes, { prefix: '/v1' })
   await app.register(moduleRoutes, { prefix: '/v1' })
+  await app.register(configRoutes, { prefix: '/v1' })
   await app.register(lookupRoutes, { prefix: '/v1' })
 
   // Module-gated routes
@@ -146,6 +149,16 @@ export async function buildApp() {
     } catch (err) {
       app.log.error({ err }, 'Failed to load module cache')
     }
+    try {
+      await AppConfigService.refreshCache(app.db)
+      const dbLogLevel = await AppConfigService.get(app.db, 'log_level')
+      if (dbLogLevel) {
+        app.log.level = dbLogLevel
+        app.log.info({ level: dbLogLevel }, 'Log level loaded from config')
+      }
+    } catch (err) {
+      app.log.error({ err }, 'Failed to load app config')
+    }
   })
 
   return app