feat: migrate to Cloudflare Workers with icon and version upload

Replace Express/SQLite and web/ with frontend/, worker/, and wrangler deploy. Add optional app icon upload, date-based app_version, and build-app workflow input.

Co-authored-by: Cursor <cursoragent@cursor.com>

worker/src/lib/response.ts

@@ -0,0 +1,17 @@
+export function jsonResponse(
+  data: unknown,
+  status = 200,
+  headers?: Record<string, string>,
+): Response {
+  return new Response(JSON.stringify(data), {
+    status,
+    headers: {
+      "Content-Type": "application/json",
+      ...headers,
+    },
+  });
+}
+
+export function errorResponse(message: string, status: number): Response {
+  return jsonResponse({ error: message }, status);
+}