BulkDetach removed one client per (email x inbound) pair, each with its own
settings rewrite, transaction and full SyncInbound. Add delInboundClients to
remove all targeted clients from an inbound in a single pass and group removals
by inbound, turning O(emails x inbounds) write cycles into O(inbounds).
BulkAttach ran the global getAllEmailSubIDs scan once per target inbound via
checkEmailsExistForClients. Compute that snapshot once per call and thread it
through a new internal addInboundClient; the duplicate check is unaffected
because attach reuses each client's existing identity (same subId).
Covered by bulk_clients_test.go: VLESS round-trip (linkage, settings JSON,
idempotency, record survival), skip-unattached, and Trojan key matching.
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