feat(observability): add exception filter, logging interceptor, and CodeExecutorModule

- add HttpExceptionFilter logging BadRequestException at warn and InternalServerErrorException at error with cause chain
- add LoggingInterceptor logging request/response pairs at debug level with method, path, body, status, and duration
- extract CodeExecutorService into standalone CodeExecutorModule imported by BrowserModule and McpModule
- thread { cause: err } into all catch blocks across auth, browser, and code-executor services

package.json

@@ -21,6 +21,7 @@
     "@nestjs/platform-express": "^11.1.18",
     "@nestjs/swagger": "^11.2.6",
     "@nestjs/typeorm": "^11.0.1",
+    "acorn": "^8.16.0",
     "better-sqlite3": "^12.8.0",
     "class-transformer": "^0.5.1",
     "class-validator": "^0.14.4",
@@ -34,6 +35,7 @@
   },
   "devDependencies": {
     "@nestjs/cli": "^11.0.17",
+    "@types/acorn": "^4.0.6",
     "@types/better-sqlite3": "^7.6.13",
     "@types/jsdom": "^28.0.1",
     "@types/mozilla__readability": "^0.4.2",