feat(fleet): Phase B — fleet overview UI + GET /api/fleet read endpoint

Tenant-scoped fleet read: GET /api/fleet returns agent_hosts (host
metrics) each with their game_instances, plus a summary
(host/instance/online counts). FleetView lists host cards (status, CPU/
mem/disk/uptime/last-heartbeat) with their instances (game, state badge,
uptime); honest empty state -> Server page when no hosts. New 'Fleet'
sidebar nav item across all four game profiles, /fleet route. Store
follows the no-throw-on-fetch pattern (error state, never bricks). The
marketing hero made real from the live fleet tables.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend-nest/src/app.module.ts

@@ -45,6 +45,7 @@ import { BetterChatModule } from './modules/betterchat/betterchat.module';
 import { TimedExecuteModule } from './modules/timedexecute/timedexecute.module';
 import { RaidableBasesModule } from './modules/raidablebases/raidablebases.module';
 import { EarlyAccessModule } from './modules/early-access/early-access.module';
+import { FleetModule } from './modules/fleet/fleet.module';
 
 // Shared Services
 import { NatsService } from './services/nats.service';
@@ -133,6 +134,7 @@ import { NatsBridgeGateway } from './gateways/nats-bridge.gateway';
     TimedExecuteModule,
     RaidableBasesModule,
     EarlyAccessModule,
+    FleetModule,
   ],
   providers: [
     // Global guards (order matters: auth first, then license, then permissions)