feat(core): implement data retention, metrics, and enhanced media handling

This commit introduces several significant improvements across the backend and gateway services:

- **Data Retention**: Added an automated cleanup scheduler in `discord-gateway` to prune expired messages, attachments, and voice recordings based on configurable retention policies.
- **Observability**: Integrated `prom-client` in the `backend` service to expose Prometheus metrics via `/api/metrics` and added default Node.js runtime metrics.
- **Media Handling**: Enhanced `MediaHandler` in `discord-gateway` to support media URL resolution and improved playback status tracking.
- **API & Config**: Expanded the configuration endpoint to expose more system settings and reorganized `.env.example` for better readability.
- **Refactoring & Cleanup**:
    - Removed unused `better-sqlite3` dependency.
    - Refactored voice channel routing.
    - Improved error handling and testing coverage with comprehensive unit tests for shared utilities and error classes.
- **Documentation**: Added `MEMORY.md` for project context.

packages/shared/package.json

@@ -11,6 +11,8 @@
     "./database/schema": "./dist/database/schema.js",
     "./errors": "./dist/errors/index.js",
     "./logger": "./dist/logger/index.js",
+    "./moderation-types": "./dist/moderation-types.js",
+    "./redis-channels": "./dist/redis-channels.js",
     "./utils": "./dist/utils/index.js"
   },
   "scripts": {