Two findings from JohnBaumb on the engine refactor.
(1) Every download client returned None when self._engine was None,
just logging an error. The orchestrator's download_with_fallback
treated None as "source declined", so the user got no feedback —
download silently disappeared. Now each client raises a RuntimeError
on the engine-not-wired path. download_with_fallback already catches
plugin exceptions, logs a warning, and tries the next source — so
the visible behavior is "real error in logs + fallback to next
source" instead of "silent drop". Six clients touched (deezer, hifi,
qobuz, soundcloud, tidal, youtube). Pinning tests updated to expect
raise.
(2) Monitor's engine.get_all_downloads() walked every plugin
including soulseek, but the same monitor loop already pulled slskd
transfers via the transfers/downloads endpoint a few lines earlier —
soulseek's records were being fetched twice per tick. Same issue in
web_server.py's get_cached_transfer_data path. Added an exclude
parameter to engine.get_all_downloads(); both call sites now pass
('soulseek',). New test pins the exclude semantic.
Also fixed a stray 8-space over-indent on the for-loop body in
get_cached_transfer_data (cosmetic, JohnBaumb flagged the same
pattern in monitor.py earlier).
