fix: upscale times out behind Cloudflare Tunnel due to blocking HTTP request

The upscale route held the HTTP connection open for the full duration of
Python sidecar processing (30-300s). Behind proxies with connection
timeouts (Cloudflare Tunnel: 100s), this caused HTTP 524 errors.

The route now returns 202 Accepted immediately after upload validation
and processes in the background. The result (downloadUrl, sizes, etc.)
is delivered via the existing SSE progress channel. The frontend detects
the 202 and waits for the SSE completion event instead of reading the
XHR response body. A reconnect-safe completion store ensures results
survive brief SSE disconnects.

Closes #106

apps/api/src/routes/progress.ts

@@ -32,11 +32,15 @@ export interface SingleFileProgress {
   stage?: string;
   percent: number;
   error?: string;
+  result?: Record<string, unknown>;
 }
 
 /** In-memory store of job progress, keyed by jobId. */
 const jobProgressStore = new Map<string, JobProgress>();
 
+/** Terminal single-file events kept for SSE reconnect replay. */
+const singleFileCompletions = new Map<string, SingleFileProgress>();
+
 /** SSE listeners waiting for updates, keyed by jobId. */
 const listeners = new Map<string, Set<(data: JobProgress | SingleFileProgress) => void>>();
 
@@ -183,6 +187,16 @@ export function updateJobProgress(progress: JobProgress): void {
 export function updateSingleFileProgress(progress: Omit<SingleFileProgress, "type">): void {
   const event: SingleFileProgress = { ...progress, type: "single" };
   persistSingleFileProgress(progress);
+
+  if (progress.phase === "complete" || progress.phase === "failed") {
+    if (singleFileCompletions.size >= 10_000) {
+      const oldest = singleFileCompletions.keys().next().value;
+      if (oldest) singleFileCompletions.delete(oldest);
+    }
+    singleFileCompletions.set(progress.jobId, event);
+    setTimeout(() => singleFileCompletions.delete(progress.jobId), 120_000);
+  }
+
   const subs = listeners.get(progress.jobId);
   if (subs) {
     for (const cb of subs) {
@@ -228,6 +242,13 @@ export async function registerProgressRoutes(app: FastifyInstance): Promise<void
         }
       }
 
+      const existingSingle = singleFileCompletions.get(jobId);
+      if (existingSingle) {
+        sendEvent(existingSingle);
+        reply.raw.end();
+        return;
+      }
+
       // Subscribe to updates
       if (!listeners.has(jobId)) {
         listeners.set(jobId, new Set());