feat(documents): folder filter on list + per-doc move endpoint

listDocuments accepts folderId (string | null | undefined) and
includeDescendants. folderId=null returns only docs at root;
includeDescendants=true expands the subtree via collectDescendantIds
(in-memory walk over the cached tree -- folder trees are small).

PATCH /api/v1/documents/[id]/folder moves a single document under
documents.manage_folders, with audit-log metadata { type: 'folder_move' }.
Bumping updatedAt is correct for per-doc moves because reps deliberately
acted on that document -- different semantics from the bulk soft-rescue
in Task 4.

createDocument accepts an optional folderId for the upcoming UI's
"create in current folder" affordance.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/services/document-folders.service.ts

@@ -275,3 +275,25 @@ export async function deleteFolderSoftRescue(
     metadata: { rescuedTo: newParent },
   });
 }
+
+/**
+ * Walk a folder tree and return the IDs of every descendant of `rootId`
+ * (NOT including rootId itself). Used by `listDocuments` when
+ * `includeDescendants=true` so a port-wide tree fetch is reused
+ * instead of a recursive CTE.
+ */
+export function collectDescendantIds(tree: FolderNode[], rootId: string): string[] {
+  const out: string[] = [];
+  function visit(nodes: FolderNode[], inside: boolean) {
+    for (const n of nodes) {
+      if (inside || n.id === rootId) {
+        if (n.id !== rootId) out.push(n.id);
+        visit(n.children, true);
+      } else {
+        visit(n.children, false);
+      }
+    }
+  }
+  visit(tree, false);
+  return out;
+}