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/config/config.routes.ts

@@ -9,6 +9,18 @@ export function createConfigRouter(): Router {
   router.get("/config", (_req, res) => {
     res.json({
       monitorGuildId: config.MONITOR_GUILD_ID || null,
+      webserverPort: config.WEBSERVER_PORT,
+      nodeEnv: config.NODE_ENV,
+      backlogSyncHours: config.BACKLOG_SYNC_HOURS,
+      backlogSyncBatchSize: config.BACKLOG_SYNC_BATCH_SIZE,
+      retentionMessagesDays: config.RETENTION_MESSAGES_DAYS,
+      retentionAttachmentsDays: config.RETENTION_ATTACHMENTS_DAYS,
+      retentionVoiceDays: config.RETENTION_VOICE_DAYS,
+      autoDeleteFlaggedEnabled: config.AUTO_DELETE_FLAGGED_ENABLED,
+      aiAnalysisEnabled: config.AI_ANALYSIS_ENABLED,
+      voiceGuildId: config.VOICE_GUILD_ID || null,
+      voiceChannelId: config.VOICE_CHANNEL_ID || null,
+      logLevel: config.LOG_LEVEL,
     });
   });
 

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;
 }

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

@@ -2,7 +2,11 @@ import { createChildLogger } from "@bete/shared/logger";
 import type { Request, Response, Router } from "express";
 import express from "express";
 import { asyncHandler } from "../../shared/middlewares/index.js";
-import { getGuilds, getTextChannels } from "./voice.service.js";
+import {
+  getGuilds,
+  getTextChannels,
+  getVoiceChannels,
+} from "./voice.service.js";
 
 const logger = createChildLogger("guilds.routes");
 
@@ -32,5 +36,18 @@ export function createGuildsRouter(): Router {
     }),
   );
 
+  // GET /api/guilds/:guildId/voice-channels
+  router.get(
+    "/:guildId/voice-channels",
+    asyncHandler(async (req: Request, res: Response) => {
+      const guildId = Array.isArray(req.params.guildId)
+        ? req.params.guildId[0]
+        : req.params.guildId;
+      logger.debug({ guildId }, "Fetching voice channels");
+      const channels = await getVoiceChannels(guildId);
+      res.json(channels);
+    }),
+  );
+
   return router;
 }

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);