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/early-access.controller.ts

@@ -0,0 +1,19 @@
+import { Body, Controller, HttpCode, HttpStatus, Post } from '@nestjs/common';
+import { ApiOperation, ApiTags } from '@nestjs/swagger';
+import { Public } from '../../common/decorators/public.decorator';
+import { EarlyAccessService } from './early-access.service';
+import { CreateEarlyAccessDto } from './dto/create-early-access.dto';
+
+@ApiTags('early-access')
+@Controller()
+export class EarlyAccessController {
+  constructor(private readonly earlyAccessService: EarlyAccessService) {}
+
+  @Public()
+  @Post('early-access')
+  @HttpCode(HttpStatus.OK)
+  @ApiOperation({ summary: 'Register for early access' })
+  async register(@Body() dto: CreateEarlyAccessDto) {
+    return this.earlyAccessService.register(dto);
+  }
+}