refactor(api): collapse provider+stream service.go into single api/service.go (Mantis #1336)

Snider correction 2026-05-01: one Service per repo, not one per
subpackage. Drops the per-subpackage pkg/provider/service.go +
pkg/stream/service.go added in ca534ca and replaces with a single
api/go/service.go in package api wrapping the live *Engine.

Action surface narrows from 10 introspection actions across two
services to 4 unified api.{addr, groups, group_count, channels}
actions. RouteGroup / Provider / StreamGroup registration stays on
direct svc.Engine method use — interface-typed values + gin handlers
do not cross IPC boundaries, and Engine's Register / RegisterStreamGroup
already cover the full surface.

provider.Registry + stream.NewGroup remain as non-Service helpers for
collecting Providers / declaring StreamGroups before mounting onto
svc.Engine.

Co-Authored-By: Cladius <noreply@anthropic.com>

Author: Snider