feat(audit): add V49 rollup covering index + raise /api/dashboard/activity cap to 40

- V49__add_audit_log_rollup_index.sql: partial covering index on
  (actor_id, document_id, kind, happened_at DESC) filtered by the 6 rollup
  kinds. Matches the WHERE clause of findRolledUpActivityFeed exactly so the
  session-grouping window scan is index-backed.
- DashboardController: clamp limit to 40 (was 20). Chronik requests up to 40
  activity items per page; dashboard side-rail still passes 7.

Part of #285.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/dashboard/DashboardController.java

@@ -37,6 +37,6 @@ public class DashboardController {
             Authentication authentication,
             @RequestParam(defaultValue = "7") int limit) {
         UUID userId = SecurityUtils.requireUserId(authentication, userService);
-        return dashboardService.getActivity(userId, Math.min(limit, 20));
+        return dashboardService.getActivity(userId, Math.min(limit, 40));
     }
 }