feat(files): add file storage subsystem with scenario and run artifact support

- add FileEntity, ScenarioFileEntity, ScenarioRunFileEntity with UUID-sharded disk tree
- FileStorageService: write/sha256/expiry, paginated listing, dynamic cleanup cron via SchedulerRegistry
- expose getScenarioFiles() and downloadFile() on ScriptContext for use in exec code
- wire scenarioId, runId, fileService into ExecContextBuilder and scenario scheduler
- add file upload and listing endpoints to ScenarioController
- add FILES_DIR, FILE_RUN_EXPIRATION_DAYS, FILE_CLEANUP_CRON to AppConfig

server/src/common/file-downloader.ts

@@ -0,0 +1,77 @@
+import * as https from "https";
+import * as http from "http";
+import { URL } from "url";
+
+const MAX_FILE_SIZE = 100 * 1024 * 1024; // 100 MB
+const TIMEOUT_MS = 30000; // 30 seconds
+
+export async function downloadFile(
+  urlStr: string,
+  options?: {
+    method?: string;
+    headers?: Record<string, string>;
+    body?: string;
+  },
+): Promise<Buffer> {
+  const url = new URL(urlStr);
+  const protocol = url.protocol === "https:" ? https : http;
+  const method = options?.method ?? "GET";
+  const headers = options?.headers ?? {};
+
+  return new Promise((resolve, reject) => {
+    const req = protocol.request(
+      {
+        hostname: url.hostname,
+        port: url.port,
+        path: url.pathname + url.search,
+        method,
+        headers,
+        timeout: TIMEOUT_MS,
+      },
+      (res) => {
+        // Follow redirects
+        if (res.statusCode && res.statusCode >= 300 && res.statusCode < 400 && res.headers.location) {
+          downloadFile(res.headers.location, options).then(resolve).catch(reject);
+          return;
+        }
+
+        if (res.statusCode && res.statusCode !== 200) {
+          reject(new Error(`HTTP ${res.statusCode}: ${res.statusMessage}`));
+          return;
+        }
+
+        const chunks: Buffer[] = [];
+        let totalSize = 0;
+
+        res.on("data", (chunk: Buffer) => {
+          totalSize += chunk.length;
+          if (totalSize > MAX_FILE_SIZE) {
+            req.destroy();
+            reject(new Error(`File exceeds maximum size of ${MAX_FILE_SIZE} bytes`));
+            return;
+          }
+          chunks.push(chunk);
+        });
+
+        res.on("end", () => {
+          resolve(Buffer.concat(chunks));
+        });
+
+        res.on("error", reject);
+      },
+    );
+
+    req.on("timeout", () => {
+      req.destroy();
+      reject(new Error("Download timeout"));
+    });
+
+    req.on("error", reject);
+
+    if (options?.body && (method === "POST" || method === "PUT")) {
+      req.write(options.body);
+    }
+
+    req.end();
+  });
+}