feat(api): early-access signup endpoint (POST /api/early-access)

Real @Public() NestJS endpoint persisting to the existing early_access_signups table (email + server_count), matching the schema exactly (no migration). Duplicate-email safe (pre-check + unique-constraint catch -> friendly success). Wired into app.module. Makes the marketing early-access form functional end-to-end on next API deploy. tsc/nest build green.

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

backend-nest/src/modules/early-access/dto/create-early-access.dto.ts

@@ -0,0 +1,14 @@
+import { IsEmail, IsOptional, IsString, MaxLength } from 'class-validator';
+import { ApiProperty, ApiPropertyOptional } from '@nestjs/swagger';
+
+export class CreateEarlyAccessDto {
+  @ApiProperty({ example: 'admin@example.com' })
+  @IsEmail()
+  email: string;
+
+  @ApiPropertyOptional({ example: 'rust', description: 'Primary game interest or server count' })
+  @IsOptional()
+  @IsString()
+  @MaxLength(10)
+  server_count?: string;
+}