feat(kol): enhance KOL constraints per review findings

- Add composite FK kol_usage_logs(prompt_id, prompt_revision_no) -> kol_prompt_revisions
  to enforce audit replay integrity
- Add CHECK constraint: active prompts must have a published revision
- Add tenant-leading index on kol_packages for list path performance

Addresses review feedback on commit 60fb8a4.

server/migrations/20260417100050_enhance_kol_constraints.down.sql

@@ -0,0 +1,7 @@
+DROP INDEX IF EXISTS idx_kol_package_tenant;
+
+ALTER TABLE kol_prompts
+    DROP CONSTRAINT IF EXISTS chk_kol_prompts_active_has_published_revision;
+
+ALTER TABLE kol_usage_logs
+    DROP CONSTRAINT IF EXISTS fk_kol_usage_logs_prompt_revision;