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/health/health.controller.ts

@@ -1,4 +1,5 @@
 import type { Request, Response } from "express";
+import { register } from "prom-client";
 import { asyncHandler } from "../../shared/middlewares/index.js";
 import { healthService } from "./health.service.js";
 
@@ -10,3 +11,10 @@ export const handleHealthCheck = asyncHandler(
     res.status(status).json(result);
   },
 );
+
+export const handleMetrics = asyncHandler(
+  async (_req: Request, res: Response) => {
+    res.set("Content-Type", register.contentType);
+    res.end(await register.metrics());
+  },
+);

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

@@ -1,6 +1,11 @@
+import { collectDefaultMetrics, register } from "prom-client";
 import type { Router } from "express";
 import express from "express";
-import { handleHealthCheck } from "./health.controller.js";
+import { handleHealthCheck, handleMetrics } from "./health.controller.js";
+
+// Initialize default Node.js runtime metrics (event loop lag, memory, GC, etc.)
+// Called once at module load, not per-request.
+collectDefaultMetrics();
 
 export function createHealthRouter(): Router {
   const router = express.Router();
@@ -8,5 +13,8 @@ export function createHealthRouter(): Router {
   // GET /api/health
   router.get("/health", handleHealthCheck);
 
+  // GET /api/metrics — Prometheus scrape endpoint
+  router.get("/metrics", handleMetrics);
+
   return router;
 }