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/migrations/meta/_journal.json

@@ -281,6 +281,13 @@
       "when": 1775408000000,
       "tag": "0039_cash-rounding",
       "breakpoints": true
+    },
+    {
+      "idx": 40,
+      "version": "7",
+      "when": 1775494000000,
+      "tag": "0040_app-config",
+      "breakpoints": true
     }
   ]
 }