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/modules/fleet/fleet.module.ts

@@ -0,0 +1,14 @@
+import { Module } from '@nestjs/common';
+import { TypeOrmModule } from '@nestjs/typeorm';
+import { FleetController } from './fleet.controller';
+import { FleetService } from './fleet.service';
+import { AgentHost } from '../../entities/agent-host.entity';
+import { GameInstance } from '../../entities/game-instance.entity';
+
+@Module({
+  imports: [TypeOrmModule.forFeature([AgentHost, GameInstance])],
+  controllers: [FleetController],
+  providers: [FleetService],
+  exports: [FleetService],
+})
+export class FleetModule {}