Cin's review feedback: external callers reach per-source clients
via attribute access (orch.hifi.reload_instances()) — needs
generic accessors so the registry IS the single source of truth.

Adds:
- orch.client(name) — public accessor for a per-source client.
  Resolves canonical names (deezer) AND legacy aliases (deezer_dl).
- orch.configured_clients() — returns {name: client} for every
  initialized AND is_configured() == True source. Replaces the
  6+ if/hasattr/is_configured chain Cin called out:
    if hasattr(orch, 'soulseek') and orch.soulseek and \
       orch.soulseek.is_configured(): ...
- orch.reload_instances(source=None) — generic dispatch for
  source-specific reload calls. Replaces orch.hifi.reload_instances()
  with orch.reload_instances('hifi').
- get_download_orchestrator() / set_download_orchestrator()
  singleton factory matching Cin's get_metadata_engine pattern in
  PR #498. web_server.py can install the orchestrator it builds
  at boot so future callers grab via the factory instead of
  importing the legacy `soulseek_client` global.

Phase Cin-3/Cin-4 will replace existing call sites; this commit
just provides the surface so those migrations are mechanical.

Suite still green (335 download tests + 6 new generic-accessor
tests).