perf(db): add missing indexes, fix N+1 batch delete, add pagination limits

- Add indexes on Resource(blueprintId, roleId), DemandRequirement(roleId),
  Assignment(roleId) — commonly filtered FK columns that were missing indexes
- Replace N+1 batch delete pattern (2N queries) with findAllocationEntries()
  that does 2 total queries via findMany({ id: { in: ids } })
- Add take/skip pagination with default limit of 500 to listDemands and
  listAssignments to prevent unbounded result sets

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

packages/db/prisma/schema.prisma

@@ -914,6 +914,8 @@ model Resource {
   @@index([orgUnitId])
   @@index([resourceType])
   @@index([managementLevelGroupId])
+  @@index([blueprintId])
+  @@index([roleId])
   @@map("resources")
 }
 
@@ -1323,6 +1325,7 @@ model DemandRequirement {
   @@index([startDate, endDate])
   @@index([status])
   @@index([projectId, status, startDate])
+  @@index([roleId])
   @@map("demand_requirements")
 }
 
@@ -1362,6 +1365,7 @@ model Assignment {
   @@index([projectId, status, startDate, endDate])
   @@index([resourceId, status, endDate])
   @@index([projectId, status, endDate])
+  @@index([roleId])
   @@map("assignments")
 }