fix: increase relay token TTL from 5 minutes to 3 hours

The 5-minute default caused unnecessary reconnect cycles every 5
minutes, disrupting sessions and generating extra relay container
traffic. The token is only used for WebSocket authentication on
connect, so a longer TTL has no security downside.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: DJBen

backend/functions/src/config.ts

@@ -22,7 +22,7 @@ export function getConfig(): RuntimeConfig {
     relayTokenSecret: process.env.RELAY_TOKEN_SECRET || process.env.SESSION_TOKEN_SECRET || "dev-relay-secret",
     turnSecret: process.env.TURN_SECRET || "dev-turn-secret",
     sessionTtlSeconds: Number(process.env.SESSION_TTL_SECONDS || 3600),
-    relayTokenTtlSeconds: Number(process.env.RELAY_TOKEN_TTL_SECONDS || 300),
+    relayTokenTtlSeconds: Number(process.env.RELAY_TOKEN_TTL_SECONDS || 10800),
     graceTtlSeconds: Number(process.env.GRACE_TTL_SECONDS || 180)
   };
 }