feat(frontend): add System tab to admin panel with backfill-versions action

Admin can trigger an initial history snapshot for all documents without
version history. Shows count of backfilled documents after completion.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/generated/api.ts

@@ -228,6 +228,22 @@ export interface paths {
         patch?: never;
         trace?: never;
     };
+    "/api/admin/backfill-versions": {
+        parameters: {
+            query?: never;
+            header?: never;
+            path?: never;
+            cookie?: never;
+        };
+        get?: never;
+        put?: never;
+        post: operations["backfillVersions"];
+        delete?: never;
+        options?: never;
+        head?: never;
+        patch?: never;
+        trace?: never;
+    };
     "/api/groups/{id}": {
         parameters: {
             query?: never;
@@ -548,6 +564,10 @@ export interface components {
             /** Format: date-time */
             startedAt?: string;
         };
+        BackfillResult: {
+            /** Format: int32 */
+            count: number;
+        };
         DocumentVersionSummary: {
             /** Format: uuid */
             id: string;
@@ -1106,6 +1126,26 @@ export interface operations {
             };
         };
     };
+    backfillVersions: {
+        parameters: {
+            query?: never;
+            header?: never;
+            path?: never;
+            cookie?: never;
+        };
+        requestBody?: never;
+        responses: {
+            /** @description OK */
+            200: {
+                headers: {
+                    [name: string]: unknown;
+                };
+                content: {
+                    "*/*": components["schemas"]["BackfillResult"];
+                };
+            };
+        };
+    };
     deleteGroup: {
         parameters: {
             query?: never;