fix(worker): scale slskd download backstop to album size (stop abandoning big albums)

The absolute download backstop was a flat _MAX_XFER_POLLS=1200 (~60min). A large
album (e.g. Drake "Scorpion", 521MB / 25 FLACs) from a slow-but-healthy serial
peer (~400KB/s, serving one file at a time) needs ~60-70min, so it was cut off
at 60min and — because an abandoned transfer is cancelled and re-enqueued from
scratch — re-downloaded from zero every attempt, never finishing and eventually
going needs_attention at the attempt cap.

Replace the flat cap with _max_polls(total_bytes): budget the backstop at a
conservative floor throughput (~64KB/s), clamped to [20min, 2h]. Scorpion now
gets the full 2h ceiling instead of 60min. The 90s stall timeout is unchanged,
so a truly dead/queued peer is still abandoned fast.

Follow-up (not in this change): resume across retries (skip files slskd already
completed) so an abandoned large download doesn't restart from zero.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

worker/lyra_worker/adapters/_slskd.py

@@ -12,9 +12,25 @@ _DEFAULT_DOWNLOADS_ROOT = "/slskd-downloads"
 _AUDIO_EXT = {"flac", "mp3", "m4a", "ogg", "opus", "wav", "aac"}
 _LOSSLESS_EXT = {"flac", "wav"}
 _SEARCH_POLLS = 40    # * 3s ≈ 2 min
-_MAX_XFER_POLLS = 1200  # * 3s ≈ 60 min absolute backstop for a slow-but-progressing peer
 _STALL_POLLS = 30     # * 3s ≈ 90s of no byte progress → abandon a queued/dead peer and fall through
 _POLL_SECONDS = 3
+# Absolute backstop for a slow-but-progressing peer. Scaled to the album's total bytes (a flat
+# cap starved big albums: a 500MB set from a ~400KB/s serial peer needs ~60-70min and was cut at
+# a flat 60min, then re-downloaded from scratch — never finishing). Budget at a conservative floor
+# throughput, clamped. The 90s stall timeout still abandons a truly dead/queued peer quickly.
+_MIN_XFER_POLLS = 400        # * 3s ≈ 20 min floor (small albums)
+_MAX_XFER_POLLS = 2400       # * 3s ≈ 2 h ceiling (bounds worst-case slot hold)
+_BACKSTOP_FLOOR_BPS = 64_000  # budget the backstop assuming >= ~64 KB/s sustained
+
+
+def _max_polls(total_bytes: int) -> int:
+    """Poll budget (each _POLL_SECONDS) for the absolute download backstop, scaled to album size at
+    a conservative floor throughput so a large slow-but-healthy transfer isn't abandoned mid-flight.
+    Clamped to [_MIN_XFER_POLLS, _MAX_XFER_POLLS]. Pure — unit-tested."""
+    if total_bytes <= 0:
+        return _MIN_XFER_POLLS
+    budget = int(total_bytes / (_BACKSTOP_FLOOR_BPS * _POLL_SECONDS))
+    return max(_MIN_XFER_POLLS, min(budget, _MAX_XFER_POLLS))
 
 
 def _basename(path: str) -> str:
@@ -242,7 +258,7 @@ class SlskdClient:
         speed = 0.0  # bytes/sec, EWMA-smoothed so the ETA doesn't jitter poll to poll
         prev_bytes = 0
         prev_t = time.monotonic()
-        for _ in range(_MAX_XFER_POLLS):
+        for _ in range(_max_polls(total_bytes)):
             time.sleep(_POLL_SECONDS)
             data = self._get(f"/api/v0/transfers/downloads/{username}")
             states: list[str] = []