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.

package.json

@@ -22,8 +22,6 @@
   },
   "devDependencies": {
     "@biomejs/biome": "latest",
-    "@types/better-sqlite3": "^7.6.13",
-    "better-sqlite3": "^11.9.1",
     "drizzle-kit": "^0.31.10",
     "tsx": "^4.22.2",
     "typescript": "^5.9.3"