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;

server/migrations/20260417100050_enhance_kol_constraints.up.sql

@@ -0,0 +1,18 @@
+-- Enhance KOL core tables with integrity constraints and missing index.
+-- Follow-up to 20260417100000 addressing review findings:
+--   1. Composite FK for usage-log audit replay (prompt_id, prompt_revision_no)
+--   2. CHECK: active prompts must have a published revision
+--   3. Tenant-leading index on kol_packages for list paths
+
+ALTER TABLE kol_usage_logs
+    ADD CONSTRAINT fk_kol_usage_logs_prompt_revision
+    FOREIGN KEY (prompt_id, prompt_revision_no)
+    REFERENCES kol_prompt_revisions(prompt_id, revision_no);
+
+ALTER TABLE kol_prompts
+    ADD CONSTRAINT chk_kol_prompts_active_has_published_revision
+    CHECK (status <> 'active' OR published_revision_no IS NOT NULL);
+
+CREATE INDEX idx_kol_package_tenant
+    ON kol_packages(tenant_id, created_at DESC)
+    WHERE deleted_at IS NULL;