fix(jobs): make timeout messages tool-agnostic and CPU-aware (#596)

Replace the job timeout message that hardcoded "background-removal" for
every tool with a tool-agnostic one that sets the CPU-vs-GPU expectation,
the usual reason heavy AI times out on modest hardware. The client-side
SSE stall message gets the same treatment. Both stay under friendlyError's
280-char limit so the guidance reaches the user instead of collapsing to a
generic "Processing failed".

Refs #591

apps/api/src/lib/timeout.ts

@@ -24,3 +24,22 @@ export function computeExternalToolTimeout(megapixels: number): number {
   }
   return Math.max(60_000, megapixels * TIMEOUT_RATES.external * 1000);
 }
+
+/**
+ * User-facing message for a job that exceeded its worker timeout. Tool-agnostic
+ * on purpose: the previous copy hardcoded "background-removal", so an AI upscale
+ * that timed out on a modest CPU told the user a background-removal model was
+ * still downloading. Set the CPU-vs-GPU expectation instead, which is the usual
+ * reason heavy AI times out on self-hosted hardware (#591).
+ */
+export function timeoutMessage(timeoutMs: number): string {
+  const seconds = Math.round(timeoutMs / 1000);
+  // Must stay under friendlyError's 280-char / 3-line limit, or the whole
+  // message collapses to the generic "Processing failed" fallback
+  // (worker-timeout.test.ts guards this).
+  return (
+    `Timed out after ${seconds}s. Heavy tools run much slower on CPU than a GPU, ` +
+    `so a large input can exceed the limit; on the first run the model may still ` +
+    `be downloading. Try a smaller input or retry.`
+  );
+}