Fix media post failure by reading the real presigned-url response shape

Image and video posts failed at the presigned-url step: the reader looked
for the upload slot under data keys files/urls/attachments/presignedUrls but
Minnect returns it under `uploads` ({ data: { uploads: [{ attachmentId,
presignedUrl, s3Key, index }] } }), so no slot was found and every media
post returned post_failed.

Read the `uploads` key (under data or at the root), add per-step pipeline
logging that names the failing step with its status and message (never a
token, URL or bytes), and add a regression test for the real response shape.

tests/post-media.test.ts

@@ -100,6 +100,32 @@ describe("POST /api/posts (image)", () => {
     expect(calls.filter((c) => c.url.includes("/api/v1/")).every((c) => c.auth === "Bearer T")).toBe(true);
   });
 
+  it("reads the real { data: { uploads: [{ presignedUrl, ... }] } } shape", async () => {
+    const sid = session();
+    const calls: { url: string; method: string }[] = [];
+    const fn = vi.fn(async (url: string | URL, init?: RequestInit) => {
+      const u = String(url);
+      calls.push({ url: u, method: String(init?.method ?? "GET") });
+      if (u.endsWith("/api/v1/posts")) return jsonRes({ data: { id: "p9", status: "draft" } }, 201);
+      if (u.endsWith("/attachments/presigned-urls"))
+        return jsonRes(
+          { data: { uploads: [{ attachmentId: "a1", presignedUrl: "https://s3.example/put?sig=9", s3Key: "k/1.jpg", index: 0 }] } },
+          200,
+        );
+      if (u === "https://s3.example/put?sig=9") return new Response(null, { status: 200 });
+      if (u.endsWith("/attachments/confirm-uploads")) return jsonRes({ data: { success: true } }, 200);
+      if (u.includes("/publish")) return jsonRes({ data: { success: true } }, 200);
+      return new Response("{}", { status: 404 });
+    });
+    const res = await withFetch(fn as unknown as typeof fetch, () =>
+      POST(multipart("look", imageFile(), sid)),
+    );
+    expect(res.status).toBe(200);
+    expect(await res.json()).toEqual({ id: "p9" });
+    const put = calls.find((c) => c.method === "PUT");
+    expect(put?.url).toBe("https://s3.example/put?sig=9");
+  });
+
   it("never returns the feed token to the browser", async () => {
     const sid = session();
     const fn = vi.fn(async (url: string | URL) => {