feat(19-04): add users.handle, HandleGenerator and user.Created

Generate a unique mention handle on API register and admin create, and fire user.Created from both paths so listeners run in the same write.

Co-authored-by: Cursor <cursoragent@cursor.com>

updates/user_groups_test.go

@@ -54,8 +54,8 @@ func TestUserGroupsMigration(t *testing.T) {
 		t.Fatalf("users_groups primary key = %q", pk)
 	}
 
-	// OAuth identities, frontend permissions, last seen, API tokens, user permissions, then user groups.
-	for range 6 {
+	// Handle, OAuth identities, frontend permissions, last seen, API tokens, user permissions, then user groups.
+	for range 7 {
 		if err := m.RollbackLast(); err != nil {
 			t.Fatal(err)
 		}