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/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")
 }