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_composite_indexes.sql

@@ -0,0 +1,4 @@
+-- Migration: Add composite indexes on Assignment for capacity range queries
+
+CREATE INDEX IF NOT EXISTS "assignments_resourceId_status_endDate_idx" ON "assignments"("resourceId", "status", "endDate");
+CREATE INDEX IF NOT EXISTS "assignments_projectId_status_endDate_idx" ON "assignments"("projectId", "status", "endDate");

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;

packages/db/prisma/schema.prisma

@@ -1330,7 +1330,7 @@ model Assignment {
   status              AllocationStatus  @default(PROPOSED)
   metadata            Json              @db.JsonB @default("{}")
 
-  demandRequirement   DemandRequirement? @relation(fields: [demandRequirementId], references: [id])
+  demandRequirement   DemandRequirement? @relation(fields: [demandRequirementId], references: [id], onDelete: SetNull)
   resource            Resource           @relation(fields: [resourceId], references: [id])
   project             Project            @relation(fields: [projectId], references: [id])
   roleEntity          Role?              @relation(fields: [roleId], references: [id])
@@ -1346,6 +1346,8 @@ model Assignment {
   @@index([status])
   @@index([resourceId, status, startDate])
   @@index([projectId, startDate, endDate])
+  @@index([resourceId, status, endDate])
+  @@index([projectId, status, endDate])
   @@map("assignments")
 }