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/discord-gateway/src/modules/command-handler/media.handler.ts

@@ -1,15 +1,23 @@
 import { type CommandMessage, type CommandReply } from "@bete/shared";
 import { createChildLogger } from "@bete/shared/logger";
+import { StreamType } from "@discordjs/voice";
+import { resolveMediaUrl } from "../voice-recording/mediaSource.js";
 import { discordPlayer } from "../voice-recording/player.js";
 
 // ---------------------------------------------------------------------------
 // Types
 // ---------------------------------------------------------------------------
 
+interface CurrentTrack {
+  title: string;
+  url: string;
+  duration?: number;
+}
+
 export interface MediaStatusPayload {
   playing: boolean;
   musicVolume: number;
-  current: unknown;
+  current: CurrentTrack | null;
   queue: unknown[];
 }
 
@@ -19,29 +27,84 @@ export interface MediaStatusPayload {
 
 export class MediaHandler {
   private logger = createChildLogger("media-handler");
+  private currentTrack: CurrentTrack | null = null;
 
   getCurrentMediaStatus(): MediaStatusPayload {
     return {
       playing: discordPlayer.getStatus() === "playing",
       musicVolume: discordPlayer.getMusicVolume(),
-      current: null,
+      current: this.currentTrack,
       queue: [],
     };
   }
 
   async handleMediaQueue(cmd: CommandMessage): Promise<CommandReply<unknown>> {
-    this.logger.info(
-      "media:queue received — media queueing is handled externally",
-    );
-    return {
-      id: cmd.id,
-      success: true,
-      data: this.getCurrentMediaStatus(),
-    };
+    const url = String(cmd.payload.url ?? "").trim();
+
+    if (!url) {
+      this.logger.warn("media:queue received without a URL");
+      return {
+        id: cmd.id,
+        success: false,
+        data: null,
+        error: "url is required",
+      };
+    }
+
+    if (!discordPlayer.isConnected()) {
+      this.logger.warn(
+        "media:queue attempted without an active voice connection",
+      );
+      return {
+        id: cmd.id,
+        success: false,
+        data: null,
+        error: "Not connected to a voice channel. Connect to voice first.",
+      };
+    }
+
+    try {
+      this.logger.info({ url }, "Resolving media URL");
+
+      const resolution = await resolveMediaUrl(url);
+
+      this.currentTrack = {
+        title: resolution.title ?? url,
+        url,
+        duration: resolution.duration,
+      };
+
+      discordPlayer.playStream(resolution.stream, "music", {
+        inputType: StreamType.Arbitrary,
+        inlineVolume: true,
+        volume: discordPlayer.getMusicVolume(),
+      });
+
+      this.logger.info(
+        { url, title: resolution.title },
+        "Media queued and playback started",
+      );
+
+      return {
+        id: cmd.id,
+        success: true,
+        data: this.getCurrentMediaStatus(),
+      };
+    } catch (err) {
+      const message = err instanceof Error ? err.message : String(err);
+      this.logger.error({ error: message, url }, "Failed to queue media");
+      return {
+        id: cmd.id,
+        success: false,
+        data: null,
+        error: message,
+      };
+    }
   }
 
   async handleMediaSkip(cmd: CommandMessage): Promise<CommandReply<unknown>> {
     discordPlayer.stop("music");
+    this.currentTrack = null;
     return {
       id: cmd.id,
       success: true,
@@ -51,6 +114,7 @@ export class MediaHandler {
 
   async handleMediaStop(cmd: CommandMessage): Promise<CommandReply<unknown>> {
     discordPlayer.stop("music");
+    this.currentTrack = null;
     return {
       id: cmd.id,
       success: true,