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.

services/backend/src/modules/voice/voice.routes.ts

@@ -3,7 +3,6 @@ import express from "express";
 import {
   handleConnectVoice,
   handleDisconnectVoice,
-  handleGetVoiceChannels,
   handleGetVoiceStatus,
   handleVoiceCommand,
 } from "./voice.controller.js";
@@ -20,9 +19,6 @@ export function createVoiceRouter(): Router {
   // POST /api/disconnect
   router.post("/disconnect", handleDisconnectVoice);
 
-  // GET /api/guilds/:guildId/voice-channels
-  router.get("/guilds/:guildId/voice-channels", handleGetVoiceChannels);
-
   // POST /api/voice/command — send arbitrary voice command (transmit start/stop)
   router.post("/voice/command", handleVoiceCommand);