fix(metrics): convert PostgreSQL COUNT bigint to number

PostgreSQL COUNT() returns bigint type which Drizzle returns as a string.
This caused prom-client Gauge.set() to reject the value with error:
"TypeError: Value is not a valid number: 0"

Explicitly convert to Number to ensure prometheus metric accepts the value.

Related: BARAAA-64 - DB migrations and connection fixes

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/services/metrics-collector.ts

@@ -17,7 +17,7 @@ async function collectRoomsActiveMetric(pool: Pool): Promise<void> {
       .select({ count: sql<number>`count(distinct ${roomMembers.roomId})` })
       .from(roomMembers);
 
-    const count = result[0]?.count || 0;
+    const count = Number(result[0]?.count) || 0;
     roomsActiveGauge.set(count);
 
     const duration = (performance.now() - startTime) / 1000;