chore: centralize configuration management using a new Config class

- Replaced all direct `process.env` references with `Config` class constants.
- Added `dotenv` package to manage environment variables.
- Updated affected files, including schema loader, Zabbix API, resolvers, logging system, and integration points.
- Improved maintainability and consistency in environment variable handling.

package.json

@@ -24,6 +24,7 @@
     "@graphql-tools/schema": "^10.0.30",
     "class-transformer": "^0.5.1",
     "cors": "^2.8.5",
+    "dotenv": "^17.2.3",
     "graphql": "^16.12.0",
     "graphql-scalars": "^1.23.0",
     "graphql-subscriptions": "^3.0.0",