feat: add index to audit table for candidate and timestamp

Author: marc0der

src/main/resources/db/migration/V6__add_audit_index_candidate_timestamp.sql

@@ -0,0 +1,6 @@
+-- flyway:no-transaction
+
+-- Index for queries filtering by candidate with time-based ordering
+-- Use cases: download counts per candidate, recent activity for a candidate, time-series analysis
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_audit_candidate_timestamp
+ON audit(candidate, timestamp DESC);