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/resources/db/migration/V49__add_audit_log_rollup_index.sql

@@ -0,0 +1,7 @@
+-- Partial covering index for the session-style activity feed rollup (#285).
+-- Matches the WHERE clause of AuditLogQueryRepository.findRolledUpActivityFeed
+-- exactly. DESC on happened_at supports the outer ORDER BY without a sort step.
+CREATE INDEX idx_audit_log_rollup
+    ON audit_log (actor_id, document_id, kind, happened_at DESC)
+    WHERE kind IN ('TEXT_SAVED','FILE_UPLOADED','ANNOTATION_CREATED',
+                   'BLOCK_REVIEWED','COMMENT_ADDED','MENTION_CREATED');