feat(seo): static sitemap generation job via Airflow

- Backend builds sitemap.xml from school data at startup (in-memory)
- POST /api/admin/regenerate-sitemap refreshes it after data updates
- New Airflow DAG (sitemap_generate) runs Sundays 05:00 and calls the endpoint
- Next.js proxies /sitemap.xml to the backend; removes the slow dynamic sitemap.ts
- docker-compose passes BACKEND_URL + ADMIN_API_KEY to Airflow env

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nextjs-app/app/sitemap.ts

@@ -1,55 +0,0 @@
-/**
- * Dynamic Sitemap Generation
- * Generates sitemap with all school pages and main routes
- */
-
-import { MetadataRoute } from 'next';
-import { fetchSchools } from '@/lib/api';
-import { schoolUrl } from '@/lib/utils';
-
-const BASE_URL = 'https://schoolcompare.co.uk';
-
-export default async function sitemap(): Promise<MetadataRoute.Sitemap> {
-  // Static pages
-  const staticPages: MetadataRoute.Sitemap = [
-    {
-      url: BASE_URL,
-      lastModified: new Date(),
-      changeFrequency: 'daily',
-      priority: 1.0,
-    },
-    {
-      url: `${BASE_URL}/compare`,
-      lastModified: new Date(),
-      changeFrequency: 'weekly',
-      priority: 0.8,
-    },
-    {
-      url: `${BASE_URL}/rankings`,
-      lastModified: new Date(),
-      changeFrequency: 'weekly',
-      priority: 0.8,
-    },
-  ];
-
-  // Fetch all schools (in batches if necessary)
-  try {
-    const schoolsData = await fetchSchools({
-      page: 1,
-      page_size: 10000, // Fetch all schools
-    });
-
-    const schoolPages: MetadataRoute.Sitemap = schoolsData.schools.map((school) => ({
-      url: `${BASE_URL}${schoolUrl(school.urn, school.school_name)}`,
-      lastModified: new Date(),
-      changeFrequency: 'monthly',
-      priority: 0.6,
-    }));
-
-    return [...staticPages, ...schoolPages];
-  } catch (error) {
-    console.error('Failed to generate sitemap:', error);
-    // Return just static pages if school fetch fails
-    return staticPages;
-  }
-}