feat(artifacts): capture Claude Docs docs and session documents, claim unowned rows

- A Claude Docs `batch` that creates a doc is captured like an `Artifact`
  publish, titled by the doc's name and linked by its own frame URL.
- At Stop/SessionEnd, each pptx/docx/xlsx/pdf/html a workspace session
  created or changed is snapshotted into its owner's library (`auto`);
  an unchanged file is skipped, a changed one refreshes its snapshot.
- Server mode: a page with no owner carries a suggested owner (the
  workspace or home its session ran in). The suggested user claims or
  declines it; an admin claims or assigns any. New tab Unowned /
  Is this yours? on the Artifacts page; all three actions audited.

server/routes/artifacts.js

@@ -38,6 +38,7 @@ const STATUS = {
   EMARKETFETCH: 502,
   ENEEDLOGIN: 502,
   ETOOBIG: 413,
+  EOWNED: 409,
 };
 
 function sendError(res, err) {
@@ -362,6 +363,34 @@ router.post(
   })
 );
 
+// A row nobody owns: its suggested user says "mine" or "not mine"; an admin
+// takes it or gives it to someone (artifacts.js `claim`/`decline`/`assign`).
+router.post(
+  "/:id/claim",
+  sameOriginGuard,
+  wrap((req, res) => {
+    const item = artifacts.claim(req, req.params.id);
+    changed(item);
+    res.json({ item });
+  })
+);
+
+router.post(
+  "/:id/decline",
+  sameOriginGuard,
+  wrap((req, res) => res.json(artifacts.decline(req, req.params.id)))
+);
+
+router.post(
+  "/:id/assign",
+  sameOriginGuard,
+  wrap((req, res) => {
+    const item = artifacts.assign(req, req.params.id, req.body?.userId);
+    changed(item);
+    res.json({ item });
+  })
+);
+
 router.post(
   "/:id/uninstall",
   sameOriginGuard,