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.

client/src/lib/api.ts

@@ -3016,7 +3016,9 @@ export const api = {
   },
   /** The Artifacts hub (`/api/artifacts`, `server/lib/artifacts.js`). */
   artifacts: {
-    list: (params: { scope?: "mine" | "hub" | "all"; kind?: ArtifactKind; q?: string } = {}) => {
+    list: (
+      params: { scope?: "mine" | "hub" | "all" | "unowned"; kind?: ArtifactKind; q?: string } = {}
+    ) => {
       const qs = new URLSearchParams();
       if (params.scope) qs.set("scope", params.scope);
       if (params.kind) qs.set("kind", params.kind);
@@ -3092,6 +3094,18 @@ export const api = {
       for (const [k, v] of Object.entries(fields)) if (v) form.append(k, v);
       return request<{ item: Artifact }>("/artifacts/file/upload", { method: "POST", body: form });
     },
+    /** An unowned row: its suggested user (or an admin) takes it. */
+    claim: (id: number) =>
+      request<{ item: Artifact }>(`/artifacts/${id}/claim`, { method: "POST" }),
+    /** An unowned row offered to the caller is not theirs. */
+    decline: (id: number) =>
+      request<{ id: number; declined: true }>(`/artifacts/${id}/decline`, { method: "POST" }),
+    /** Admin: give an unowned row to a user. */
+    assign: (id: number, userId: number) =>
+      request<{ item: Artifact }>(`/artifacts/${id}/assign`, {
+        method: "POST",
+        body: JSON.stringify({ userId }),
+      }),
     update: (
       id: number,
       body: {
@@ -4382,7 +4396,7 @@ export interface Artifact {
   market: string | null;
   /** page: the claude.ai link. */
   url: string | null;
-  files: { path: string; size: number }[];
+  files: { path: string; size: number; mtime?: number }[];
   size: number | null;
   shared: boolean;
   sharedAt: string | null;
@@ -4391,6 +4405,12 @@ export interface Artifact {
   inLibrary: boolean;
   /** List rows only: a skill/plugin already in every account the viewer's sessions run as. */
   installed?: boolean;
+  /** file: captured when its session stopped, not pinned by hand. */
+  auto?: boolean;
+  /** Server mode: nobody owns it yet. */
+  unowned?: boolean;
+  /** Unowned and offered to the caller to claim or decline. */
+  suggestedForMe?: boolean;
   createdAt: string;
   updatedAt: string;
 }