fix(activitypub): populate in_reply_to_id in Mastodon status serializer

- status.js: in_reply_to_id was always null (both branches of ternary
  returned null — TODO left unfilled). Changed to item.inReplyToId || null.
- statuses.js POST handler: timeline insert now stores inReplyToId from
  the in_reply_to_id cursor the client already sent, so own replies are
  threaded correctly in Phanpy/Elk.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/patch-ap-status-reply-id.mjs

@@ -0,0 +1,96 @@
+/**
+ * Patch: fix in_reply_to_id always being null in Mastodon status serializer.
+ *
+ * Bug:
+ *   status.js line 207:
+ *     in_reply_to_id: item.inReplyTo ? null : null, // TODO: resolve to local ID
+ *
+ *   Both branches of the ternary return null, so in_reply_to_id is ALWAYS null.
+ *   Mastodon clients (Phanpy, Elk) use this field to display reply threading —
+ *   without it, replies appear as standalone posts with no thread context.
+ *
+ * Fix (two changes):
+ *
+ *   A) status.js — use item.inReplyToId (the encoded cursor of the parent post)
+ *      instead of the tautological null.
+ *
+ *   B) statuses.js POST /api/v1/statuses handler — when pre-inserting own posts
+ *      into ap_timeline (reply-threading patch), also store
+ *      inReplyToId: inReplyToId || null
+ *      (inReplyToId is already in scope as the raw in_reply_to_id param from the
+ *      client, which IS a valid encodeCursor value.)
+ *
+ * Note: inbound AP replies from remote servers will still have inReplyToId = null
+ * until a separate patch populates it from ap_timeline lookups. Own replies via
+ * the Mastodon client API are fully fixed by this patch.
+ */
+
+import { access, readFile, writeFile } from "node:fs/promises";
+
+const MARKER = "// [patch] ap-status-reply-id";
+
+// ── Change A: fix tautological null in status.js ──────────────────────────────
+
+const statusEntityCandidates = [
+  "node_modules/@rmdes/indiekit-endpoint-activitypub/lib/mastodon/entities/status.js",
+  "node_modules/@indiekit/indiekit/node_modules/@rmdes/indiekit-endpoint-activitypub/lib/mastodon/entities/status.js",
+];
+
+const OLD_TAUTOLOGY = `    in_reply_to_id: item.inReplyTo ? null : null, // TODO: resolve to local ID`;
+const NEW_REPLY_ID  = `    in_reply_to_id: item.inReplyToId || null, ${MARKER}`;
+
+// ── Change B: store inReplyToId in the Mastodon API timeline insert ───────────
+
+const statusesRouteCandidates = [
+  "node_modules/@rmdes/indiekit-endpoint-activitypub/lib/mastodon/routes/statuses.js",
+  "node_modules/@indiekit/indiekit/node_modules/@rmdes/indiekit-endpoint-activitypub/lib/mastodon/routes/statuses.js",
+];
+
+const OLD_REPLY_INSERT = `        inReplyTo: inReplyTo || null, // [patch] ap-mastodon-reply-threading`;
+const NEW_REPLY_INSERT = `        inReplyTo: inReplyTo || null, // [patch] ap-mastodon-reply-threading
+        inReplyToId: inReplyToId || null, ${MARKER}`;
+
+async function exists(p) {
+  try { await access(p); return true; } catch { return false; }
+}
+
+async function applyPatch(candidates, oldSnippet, newSnippet, label) {
+  let checked = 0;
+  let patched = 0;
+
+  for (const filePath of candidates) {
+    if (!(await exists(filePath))) continue;
+    checked++;
+
+    const source = await readFile(filePath, "utf8");
+    if (source.includes(MARKER)) {
+      console.log(`[postinstall] patch-ap-status-reply-id: ${label} already applied to ${filePath}`);
+      continue;
+    }
+
+    if (!source.includes(oldSnippet)) {
+      console.warn(`[postinstall] patch-ap-status-reply-id: ${label} snippet not found in ${filePath}`);
+      continue;
+    }
+
+    await writeFile(filePath, source.replace(oldSnippet, newSnippet), "utf8");
+    patched++;
+    console.log(`[postinstall] Applied patch-ap-status-reply-id (${label}) to ${filePath}`);
+  }
+
+  return { checked, patched };
+}
+
+const a = await applyPatch(statusEntityCandidates, OLD_TAUTOLOGY, NEW_REPLY_ID, "status entity");
+const b = await applyPatch(statusesRouteCandidates, OLD_REPLY_INSERT, NEW_REPLY_INSERT, "timeline insert");
+
+const totalChecked = a.checked + b.checked;
+const totalPatched = a.patched + b.patched;
+
+if (totalChecked === 0) {
+  console.log("[postinstall] patch-ap-status-reply-id: no target files found");
+} else if (totalPatched === 0) {
+  console.log("[postinstall] patch-ap-status-reply-id: already up to date");
+} else {
+  console.log(`[postinstall] patch-ap-status-reply-id: patched ${totalPatched} file(s)`);
+}