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/db.js

@@ -960,6 +960,20 @@ try {
 } catch {
   db.prepare("ALTER TABLE artifacts ADD COLUMN in_library INTEGER NOT NULL DEFAULT 1").run();
 }
+// `auto = 1`: a document file a session wrote, captured when it stopped
+// (`captureOutputs`), not pinned by hand. `suggested_owner_id`: for a row with
+// no owner, the user whose folder the session ran in — they may claim it or
+// say it is not theirs (`claim`/`decline`); an admin assigns any (`assign`).
+for (const [col, ddl] of [
+  ["auto", "auto INTEGER NOT NULL DEFAULT 0"],
+  ["suggested_owner_id", "suggested_owner_id INTEGER"],
+]) {
+  try {
+    db.prepare(`SELECT ${col} FROM artifacts LIMIT 1`).get();
+  } catch {
+    db.prepare(`ALTER TABLE artifacts ADD COLUMN ${ddl}`).run();
+  }
+}
 
 // One row per install of an artifact (server/lib/artifacts.js `install`):
 // who, when, and the folders it was written to — `artifacts.installs` only