feat: implement snapshot restore functionality and enhance graph state management

src/lib/db.js

@@ -284,6 +284,60 @@ export function setSetting(key, value) {
   return pbUpsert('settings', `key="${key}"`, { value: String(value) }, { key, value: String(value) });
 }
 
+// ── Graph Snapshots ───────────────────────────────────────────────────────────
+//
+// A single "latest" snapshot is persisted in the settings collection before
+// every bulk graph write. This survives browser/tab close and lets the admin
+// roll back an AI analysis that produced unwanted results.
+//
+// The value is stored as a JSON string because the settings collection only
+// has a plain text `value` field. Large graphs (150+ topics) can produce
+// ~100 KB of JSON, which SQLite handles without issue.
+
+const SNAPSHOT_KEY = 'graph_snapshot:latest';
+
+/**
+ * Persist the current graph state as a rollback point.
+ * Called by useGraphData.bulkSave() before overwriting the graph.
+ *
+ * @param {object[]} topics    — the graph state to preserve
+ * @param {object[]} relations — the graph state to preserve
+ */
+export async function saveGraphSnapshot(topics, relations) {
+  const payload = JSON.stringify({
+    ts: Date.now(),
+    topicCount: topics.length,
+    relationCount: relations.length,
+    topics,
+    relations,
+  });
+  return setSetting(SNAPSHOT_KEY, payload);
+}
+
+/**
+ * Read the latest snapshot back from PocketBase.
+ * Returns null when no snapshot exists or the stored value is malformed.
+ *
+ * @returns {{ ts: number, topicCount: number, relationCount: number, topics: object[], relations: object[] } | null}
+ */
+export async function getGraphSnapshot() {
+  const raw = await getSetting(SNAPSHOT_KEY, null);
+  if (!raw) return null;
+  try {
+    return JSON.parse(raw);
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Delete the stored snapshot (e.g. after a successful restore so the
+ * admin cannot accidentally restore twice).
+ */
+export function clearGraphSnapshot() {
+  return setSetting(SNAPSHOT_KEY, '');
+}
+
 // ── Curriculum Versions (v2) ──────────────────────────────────────────────────
 
 export async function getCurriculumVersions(status) {