feat: implement all real backend endpoints, Redis pub/sub bridge, and gateway command handler

Backend (real implementations, no more stubs):
- Redis pub/sub bridge: subscribes to discord-gateway events (message/attachment/voice) and broadcasts to WS clients
- Redis command channel: backend publishes voice/media commands, discord-gateway executes and replies
- Voice service: connectVoice/disconnectVoice/getVoiceStatus via Redis commands with graceful fallback
- Media service: queue/skip/stop/volume via Redis commands, reads status from Redis cache
- Messages repository: ALL 7 methods now use real PostgreSQL queries (findMany, findById, findByChannel, create, update, delete, getAttachmentsByChannel)
- Analytics: period returns {start,end} epoch millis, overview includes hourly/topics/top_users, worst_flags as string[]
- Health check: actually queries SELECT 1 against database
- VoiceStatus type fixed: {connected, activeGuildId, activeChannelId, activeChannelName}
- Guild type: includes icon: string | null
- asyncHandler: accepts Promise<unknown> instead of Promise<void>

Discord Gateway:
- CommandHandler: subscribes to 'backend:command' Redis channel, executes voice/media commands, publishes replies
- Publishes voice:status and media:status to Redis for backend caching
- Shutdown handler updated to close command handler

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -1,3 +1,4 @@
+import { getPool } from "../../shared/database/index.js";
 import { createChildLogger } from "../../shared/logger/index.js";
 
 const logger = createChildLogger("health.repository");
@@ -5,11 +6,15 @@ const logger = createChildLogger("health.repository");
 export class HealthRepository {
   async checkDatabaseConnection() {
     try {
-      // TODO: Implement actual health check
+      logger.debug("Running database health check");
+      const pool = getPool();
+      await pool.query("SELECT 1 AS result");
+      logger.debug("Database health check passed");
       return { connected: true };
-    } catch (err) {
-      logger.error({ err }, "Database health check failed");
-      return { connected: false };
+    } catch (err: unknown) {
+      const message = err instanceof Error ? err.message : String(err);
+      logger.error({ error: message }, "Database health check failed");
+      return { connected: false, error: message };
     }
   }
 }