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/frontend/src/features/messages/index.tsx

@@ -1,7 +1,7 @@
 import { motion } from "framer-motion";
 import { Filter, RotateCw, Search, X } from "lucide-react";
 import { useMemo, useState } from "react";
-import type { MessageRecord } from "../../shared/api/client";
+import { type MessageRecord, request } from "../../shared/api/client";
 import { cardItem, cardStagger } from "../../shared/hooks/useFramerStagger";
 import {
   Badge,
@@ -58,9 +58,9 @@ export function MessagesPanel({
     setIsSearching(true);
     try {
       const params = new URLSearchParams({ q: searchQuery, limit: "50" });
-      const response = await fetch(`/api/analysis/search?${params}`);
-      if (!response.ok) throw new Error("Search failed");
-      const data = await response.json();
+      const data = await request<{ results: MessageRecord[] }>(
+        `/api/analysis/search?${params}`,
+      );
       setSearchResults(data.results || []);
       setShowSearch(true);
     } catch {