feat(api): outbound webhooks — server_down + player_banned events

Roadmap 'Webhook events': per-license outbound webhooks with HMAC-SHA256
signatures (X-Corrosion-Signature), 5s timeout, fire-and-forget (a webhook
failure never breaks the triggering action), last_delivery_at/last_status
tracked.

- migration 024_webhooks; Webhook entity (events as simple-array);
  WebhooksModule (@Global, exports WebhooksService) wired into app.module;
  CRUD controller (license-scoped, webhooks.view/manage).
- Hooked events: players.performAction ban -> 'player_banned';
  host-agent-consumer going-offline + staleness sweep -> 'server_down'.
- 'wipe_completed' event lands next (needs wipe status from the agent reply).

Backend tsc green. Migration applies on a fresh DB (Saturday).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend-nest/src/modules/webhooks/dto/create-webhook.dto.ts

@@ -0,0 +1,33 @@
+import { IsString, IsNotEmpty, IsUrl, IsArray, ArrayNotEmpty, IsOptional, MaxLength } from 'class-validator';
+import { ApiProperty, ApiPropertyOptional } from '@nestjs/swagger';
+
+export class CreateWebhookDto {
+  @ApiProperty({ description: 'Human-readable label for this webhook', maxLength: 100 })
+  @IsString()
+  @IsNotEmpty()
+  @MaxLength(100)
+  name: string;
+
+  @ApiProperty({ description: 'HTTPS URL to POST events to' })
+  @IsUrl({ protocols: ['https', 'http'], require_tld: false })
+  url: string;
+
+  @ApiProperty({
+    description: 'Event keys to subscribe to',
+    example: ['player_banned', 'server_down'],
+    type: [String],
+  })
+  @IsArray()
+  @ArrayNotEmpty()
+  @IsString({ each: true })
+  events: string[];
+
+  @ApiPropertyOptional({
+    description: 'HMAC-SHA256 signing secret. Auto-generated if omitted.',
+    maxLength: 128,
+  })
+  @IsOptional()
+  @IsString()
+  @MaxLength(128)
+  secret?: string;
+}