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/migrations/20260411_add_missing_fk_indexes.sql

@@ -0,0 +1,12 @@
+-- Add missing indexes on foreign key columns used in filtering/joining.
+-- These are CREATE INDEX IF NOT EXISTS so they are safe to re-run.
+
+-- Resource: blueprintId and roleId are used in resource filtering and joins
+CREATE INDEX CONCURRENTLY IF NOT EXISTS "resources_blueprintId_idx" ON "resources" ("blueprintId");
+CREATE INDEX CONCURRENTLY IF NOT EXISTS "resources_roleId_idx" ON "resources" ("roleId");
+
+-- DemandRequirement: roleId is used in role-based demand queries
+CREATE INDEX CONCURRENTLY IF NOT EXISTS "demand_requirements_roleId_idx" ON "demand_requirements" ("roleId");
+
+-- Assignment: roleId is used in role-based assignment queries
+CREATE INDEX CONCURRENTLY IF NOT EXISTS "assignments_roleId_idx" ON "assignments" ("roleId");