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/0040_app-config.sql

@@ -0,0 +1,11 @@
+CREATE TABLE IF NOT EXISTS "app_config" (
+  "key" varchar(100) PRIMARY KEY NOT NULL,
+  "value" text,
+  "description" text,
+  "updated_at" timestamp with time zone DEFAULT now() NOT NULL
+);
+
+-- Seed default log level
+INSERT INTO "app_config" ("key", "value", "description")
+VALUES ('log_level', 'info', 'Application log level (fatal, error, warn, info, debug, trace)')
+ON CONFLICT ("key") DO NOTHING;

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
     }
   ]
 }