add ALTCHA challenge endpoint and env config.

GET /api/altcha/challenge, nullcart-style env validation, and ALTCHA_ENABLED wiring.

backend/src/modules/altcha/AltchaModule.ts

@@ -0,0 +1,10 @@
+import { Module } from '@nestjs/common';
+import { AltchaController } from './controllers/AltchaController';
+import { AltchaService } from './services/AltchaService';
+
+@Module({
+    controllers: [AltchaController],
+    providers: [AltchaService],
+    exports: [AltchaService]
+})
+export class AltchaModule {}

backend/src/modules/altcha/controllers/AltchaController.ts

@@ -0,0 +1,23 @@
+import { Controller, Get, NotFoundException } from '@nestjs/common';
+import { ConfigService } from '@nestjs/config';
+import { AltchaService } from '../services/AltchaService';
+import { Config } from '../../../types/Config';
+
+@Controller('altcha')
+export class AltchaController {
+    constructor(
+        private readonly altchaService: AltchaService,
+        private readonly configService: ConfigService
+    ) {}
+
+    @Get('challenge')
+    async getChallenge() {
+        const { enabled } = this.configService.get('altcha') as Config['altcha'];
+
+        if (!enabled) {
+            throw new NotFoundException();
+        }
+
+        return this.altchaService.createChallenge();
+    }
+}

backend/src/modules/altcha/services/AltchaService.ts

@@ -0,0 +1,19 @@
+import { Injectable } from '@nestjs/common';
+import { ConfigService } from '@nestjs/config';
+import { createChallenge } from 'altcha-lib/v1';
+import type { Challenge } from 'altcha-lib/v1/types';
+import { Config } from '../../../types/Config';
+
+@Injectable()
+export class AltchaService {
+    constructor(private readonly configService: ConfigService) {}
+
+    async createChallenge(): Promise<Challenge> {
+        const { hmacKey, maxNumber } = this.configService.get('altcha') as Config['altcha'];
+
+        return createChallenge({
+            hmacKey,
+            maxNumber
+        });
+    }
+}