feat(fleet): remove host — DELETE /api/fleet/hosts/:id + Fleet card action

Self-service host removal. DELETE /api/fleet/hosts/:id (server.manage,
tenant-guarded): refuses while the host is 'connected' (409 — a live
agent re-registers on its next heartbeat, stop it first), deletes the
host's game_instances explicitly (FK is SET NULL, would otherwise
orphan them; instance_stats cascade), and clears the legacy
server_connections row if it was the license's last host. Fleet view:
offline host cards get a Remove button with inline confirm + toast.

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

backend-nest/src/modules/fleet/fleet.module.ts

@@ -4,9 +4,10 @@ import { FleetController } from './fleet.controller';
 import { FleetService } from './fleet.service';
 import { AgentHost } from '../../entities/agent-host.entity';
 import { GameInstance } from '../../entities/game-instance.entity';
+import { ServerConnection } from '../../entities/server-connection.entity';
 
 @Module({
-  imports: [TypeOrmModule.forFeature([AgentHost, GameInstance])],
+  imports: [TypeOrmModule.forFeature([AgentHost, GameInstance, ServerConnection])],
   controllers: [FleetController],
   providers: [FleetService],
   exports: [FleetService],