feat(mcpedia): Phase 4 — operability + correctness hardening

Reinterpreted from the plan's YAGNI 'Scale-out' (OpenSearch/object-storage
/multi-tenant deferred at KB scale). Phase 4 = make the Phase 3 async +
revision system correct, secure, observable, deployable.

- T1 (correctness bug): restoreRevision now rebuilds semantic chunks via new
  @mcpedia/core reindexChunks(slug) so semantic/hybrid search stay consistent
  after a restore (previously document_chunks held the NEW body while
  documents.body held the restored OLD body -> stale search).
- T2 (security): /hooks/* git-sync webhooks now require x-webhook-secret header
  matching WEBHOOK_SECRET (401 otherwise); API fails fast at startup if unset.
  Added WEBHOOK_SECRET to @mcpedia/config + .env.example; set real secret in .env.
- T3 (UX): web doc page shows a History panel (revision no/reason/date/length)
  with per-revision Restore; app/api/revisions/restore/route.ts calls
  restoreRevision + revalidatePath (server-component only, no client JS).
- T4: listRevisions gains offset paging; summary never includes body.
- T5 (ops): deploy/mcpedia-api.service + deploy/mcpedia-worker.service systemd
  units (Restart=on-failure, EnvironmentFile=.env). Not auto-enabled on host.

Verified against live imrnes Redis + Postgres: turbo typecheck+build green;
restore-rebuilds-chunks (marker present -> gone after restore); webhook 401/200;
web restore route redirects to doc + reverts body; revisions API returns summary
(no body); systemd-analyze verify passes.

packages/core/src/index.service.ts

@@ -131,6 +131,28 @@ async function snapshotRevision(
   return true;
 }
 
+/**
+ * Rebuild the semantic chunks + embeddings for a slug from its CURRENT live
+ * `documents.body`. Used after `restoreRevision` so semantic/hybrid search
+ * stay consistent with the restored body (otherwise chunks would be stale).
+ * Embed failures are logged, not thrown — FTS remains the source of truth.
+ */
+export async function reindexChunks(slug: string): Promise<number> {
+  const [doc] = await db
+    .select({ body: documents.body })
+    .from(documents)
+    .where(eq(documents.slug, slug));
+  if (!doc) return 0;
+  try {
+    return await indexChunks(slug, doc.body);
+  } catch (err) {
+    console.error(
+      `    reindexChunks embed FAILED for ${slug}: ${err instanceof Error ? err.message : err}`,
+    );
+    return 0;
+  }
+}
+
 /**
  * Walk the entire content tree and index every file. Returns aggregate counts.
  */