feat: admin image generation via Cloudflare Workers AI

- Add cfai/image.go: ImageGenClient with GenerateImage, GenerateImageFromReference, AllImageModels (9 models)
- Add handlers_image.go: GET /api/admin/image-gen/models + POST /api/admin/image-gen (JSON + multipart)
- Wire ImageGen client in main.go + server.go Dependencies
- Add admin/image-gen SvelteKit page: type toggle, model selector, prompt, reference img2img, advanced options, result panel, history, save-as-cover, download
- Add SvelteKit proxy route api/admin/image-gen forwarding to Go backend
- Add admin_nav_image_gen message key to all 5 locale files
- Add Image Gen nav link to admin layout

ui/src/routes/api/admin/image-gen/+server.ts

@@ -0,0 +1,46 @@
+/**
+ * POST /api/admin/image-gen
+ *
+ * Admin-only proxy to the Go backend's image generation endpoint.
+ * Transparently forwards the request body (JSON or multipart/form-data)
+ * and returns the JSON response containing the base64-encoded image.
+ */
+
+import { json, error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { log } from '$lib/server/logger';
+import { backendFetch } from '$lib/server/scraper';
+
+export const POST: RequestHandler = async ({ request, locals }) => {
+	if (!locals.user || locals.user.role !== 'admin') {
+		throw error(403, 'Forbidden');
+	}
+
+	const ct = request.headers.get('content-type') ?? '';
+
+	let res: Response;
+	try {
+		if (ct.includes('multipart/form-data')) {
+			// Forward raw body bytes; let the backend parse multipart
+			const body = await request.arrayBuffer();
+			res = await backendFetch('/api/admin/image-gen', {
+				method: 'POST',
+				headers: { 'content-type': ct },
+				body
+			});
+		} else {
+			const body = await request.text();
+			res = await backendFetch('/api/admin/image-gen', {
+				method: 'POST',
+				headers: { 'content-type': 'application/json' },
+				body
+			});
+		}
+	} catch (e) {
+		log.error('admin/image-gen', 'backend proxy error', { err: String(e) });
+		throw error(502, 'Could not reach backend');
+	}
+
+	const data = await res.json().catch(() => ({}));
+	return json(data, { status: res.status });
+};