Rewrite permission queries in domain_fetcher and route_fetcher to use UNION ALL

[WeiQuan0605]

• A short explanation of the proposed change:
  Rewrite the permission queries in DomainFetcher#fetch_all_for_orgs and RouteFetcher#fetch to use UNION ALL subqueries instead of OR predicates. Also adds space_ids_with_readable_routes_query to Permissions and passes readable_space_ids_dataset to RouteFetcher.fetch to avoid a guid→id round-trip.

• An explanation of the use cases your change solves
  GET /v3/domains and GET /v3/routes for non-admin users were generating correlated SubPlans in Postgres, the permission subquery was re-executed once per row in the outer result set, causing O(n²) behaviour at scale.
  The UNION ALL shape builds hash tables once and eliminates the SubPlans entirely, reducing both queries to O(n).

  GET /v3/domains (domain_fetcher)
  Benchmarked with 20,002 orgs, 101 shared domains, 390 private domains, using EXPLAIN (ANALYZE, BUFFERS)

  Shape	Avg execution time	Query plan
  OR predicate	41 ms	Seq Scan + correlated SubPlan
  UNION ALL	4.6 ms	Hash Join via Append
  Improvement	~9x faster

  GET /v3/routes (route_fetcher)
  Benchmarked with 5,001 spaces，10,002 routes，20,000 orgs, using EXPLAIN (ANALYZE, BUFFERS

  Shape	Avg execution time	Query plan
  OR predicate	5123 ms	Seq Scan + correlated SubPlan
  UNION ALL	94 ms	Hash Join via Append
  Improvement	~54x faster

• Links to any other associated PRs

  • Follows up on Fix v3/roles perf regression for regular users #5157 (Fix v3/roles perf regression for regular users)
  • Follows up on Remove id guid id roundtrips #5318 (Performance fixes for permission queries)

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• I have viewed, signed, and submitted the Contributor License Agreement

• I have made this pull request to the main branch

• I have run all the unit tests using bundle exec rake

• I have run CF Acceptance Tests