SoundCloud progress UI fix: include SoundCloud in cached transfer lookup

User: SoundCloud downloads finish correctly but the modal stays at
"Downloading... 0%" until "Processing..." flips on. Live percentage
never updates.

Root cause: my live-progress fix in 8de4a18 made the SoundCloud client
compute progress correctly via fragment_index/fragment_count — but the
percent never reached the modal because `get_cached_transfer_data` in
web_server.py iterates `[youtube, tidal, qobuz, hifi, deezer_dl,
lidarr]` to build the lookup that drives `task.progress`. SoundCloud
was missing from that loop, so `live_transfers_lookup` had no entry
for SoundCloud downloads, so `live_info` lookup at
`core/downloads/status.py:135` always missed, so `task_status['progress']`
defaulted to 0 the entire time.

Frontend was reading `task.progress` (rendered as
"Downloading... ${task.progress}%" in `webui/static/downloads.js:3142`),
which stayed at 0. The percentComplete field that the
`/api/downloads/status` endpoint includes for SoundCloud was correct;
this only affected the cached lookup used by the V2 task tracker.

Fix: include SoundCloud in the iteration. Used `getattr` fallback to
match the same pattern I used in `core/downloads/monitor.py` so older
soulseek_client snapshots without the attribute don't AttributeError.

Bonus: also wired the SoundCloud client's `set_shutdown_check` callback
in the startup block right after HiFi's. Previously the cooperative-
cancellation hook in `_progress_hook` would never fire on shutdown
because `self.shutdown_check` was None.

Verified: full suite 1732 passed, ruff clean. yt-dlp probe confirms
fragment_index / fragment_count are populated correctly during HLS
download (164 hook calls for a 19-fragment track), so the now-
exposed progress will increment smoothly from 0 to 99.9 and then
flip to Completed.

web_server.py

@@ -641,6 +641,9 @@ if soulseek_client:
     if hasattr(soulseek_client, 'hifi'):
          soulseek_client.hifi.set_shutdown_check(lambda: IS_SHUTTING_DOWN)
          logger.info("  Configured HiFi client shutdown callback")
+    if hasattr(soulseek_client, 'soundcloud') and soulseek_client.soundcloud:
+         soulseek_client.soundcloud.set_shutdown_check(lambda: IS_SHUTTING_DOWN)
+         logger.info("  Configured SoundCloud client shutdown callback")
 
 # Initialize web scan manager for automatic post-download scanning
 try:
@@ -2472,11 +2475,15 @@ def get_cached_transfer_data():
                     key = _make_context_key(transfer.get('username'), transfer.get('filename', ''))
                     live_transfers_lookup[key] = transfer
 
-            # Also add non-Soulseek downloads (avoid redundant slskd call through orchestrator)
+            # Also add non-Soulseek downloads (avoid redundant slskd call through orchestrator).
+            # Every streaming source must appear here — task progress for in-flight
+            # downloads comes from this lookup. Missing a source = task.progress
+            # stays at 0 even when the underlying client knows the real percent.
             try:
                 all_downloads = []
                 for _dl_client in [soulseek_client.youtube, soulseek_client.tidal, soulseek_client.qobuz,
-                                   soulseek_client.hifi, soulseek_client.deezer_dl, soulseek_client.lidarr]:
+                                   soulseek_client.hifi, soulseek_client.deezer_dl, soulseek_client.lidarr,
+                                   getattr(soulseek_client, 'soundcloud', None)]:
                     if _dl_client:
                         try:
                             all_downloads.extend(run_async(_dl_client.get_all_downloads()))