feat(artifacts): last updated time on skills and files; each installer's copy marked

A row said nothing about when its content last changed, and the install list
could not tell who still runs an old version. `artifacts.version_at` (new
column, existing rows from `updated_at`) is stamped only when a snapshot is
replaced — a rename or share toggle leaves it — and served as `versionAt`:
skill and file rows read "Updated 3h ago", the detail sheet "Last updated".
`GET /api/artifacts/:id/installs` gives each skill install a `state` —
latest / outdated / removed / unknown — from its Claude skill copies' hash
against the newest snapshot, shown as a badge.

client/src/lib/api.ts

@@ -4720,11 +4720,21 @@ export interface Artifact {
   suggestedForMe?: boolean;
   createdAt: string;
   updatedAt: string;
+  /** When its content last changed (a new snapshot); a page or plugin: when it was added. */
+  versionAt: string;
 }
 
 /** GET /api/artifacts/:id/installs. `paths` only for your own installs (or an admin). */
 export interface ArtifactInstalls {
-  items: { user: string | null; mine: boolean; at: string; replaced: boolean; paths?: string[] }[];
+  items: {
+    user: string | null;
+    mine: boolean;
+    at: string;
+    replaced: boolean;
+    /** Skills: where that install's copy stands against the newest version now. */
+    state?: "latest" | "outdated" | "removed" | "unknown";
+    paths?: string[];
+  }[];
   /** Installs counted before the list was kept. */
   unrecorded: number;
 }