fix(db): add SetNull cascade on Assignment→DemandRequirement + composite indexes

Prevents orphaned Assignment rows when a DemandRequirement is deleted.
Adds (resourceId, status, endDate) and (projectId, status, endDate)
indexes to support capacity range queries.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/db/prisma/migrations/20260409_assignment_demand_requirement_cascade.sql

@@ -0,0 +1,7 @@
+-- Migration: Add SetNull cascade on Assignment → DemandRequirement FK
+-- When a DemandRequirement is deleted, assignments lose their reference but are not deleted.
+
+ALTER TABLE "assignments" DROP CONSTRAINT IF EXISTS "assignments_demandRequirementId_fkey";
+ALTER TABLE "assignments" ADD CONSTRAINT "assignments_demandRequirementId_fkey"
+  FOREIGN KEY ("demandRequirementId") REFERENCES "demand_requirements"("id")
+  ON DELETE SET NULL ON UPDATE CASCADE;