fix(worker): cancel abandoned slskd transfers + cap download fall-through

Two compounding causes of 'many downloads of one album from different sources' in
slskd: (1) the slskd client raised on timeout/error WITHOUT cancelling the transfer
it enqueued, so as the pipeline fell through to other peers each left a transfer
running; (2) the incomplete-download fall-through was uncapped, grinding through
every candidate of a popular album (~200 Soulseek peers), amplified now that Qobuz
pacing pushes most jobs to Soulseek.

- SlskdClient.download now cancels its enqueued transfers (DELETE) on any abandon.
- The download loop caps at _MAX_DOWNLOAD_ATTEMPTS (6) candidates.

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

worker/lyra_worker/adapters/_slskd.py

@@ -119,7 +119,17 @@ class SlskdClient:
         files = ref["files"]
         wanted = {f["filename"] for f in files}
         os.makedirs(dest, exist_ok=True)
+        xfer_ids: set[str] = set()  # slskd transfer ids we started, for cancel-on-abandon
+        try:
+            return self._download(username, files, wanted, dest, on_progress, xfer_ids)
+        except Exception:
+            # Abandoning this peer (timeout / error / the pipeline moving on): cancel the transfers
+            # we enqueued so they don't keep running and pile up in slskd as the job falls through
+            # to other peers/sources for the same album.
+            self._cancel_transfers(username, xfer_ids)
+            raise
 
+    def _download(self, username, files, wanted, dest, on_progress, xfer_ids: set) -> dict:
         r = requests.post(
             f"{self._url}/api/v0/transfers/downloads/{username}",
             headers=self._headers(),
@@ -139,6 +149,8 @@ class SlskdClient:
                 for f in directory.get("files", []):
                     if f.get("filename") in wanted:  # only the files we enqueued
                         states.append(str(f.get("state", "")))
+                        if f.get("id"):  # remember the transfer so we can cancel it if abandoned
+                            xfer_ids.add(str(f["id"]))
             if states:
                 if any(bad in s for s in states for bad in ("Errored", "Rejected", "Cancelled")):
                     raise RuntimeError("soulseek transfer failed")
@@ -162,6 +174,19 @@ class SlskdClient:
             )
         return {"track_count": moved, "path": dest}
 
+    def _cancel_transfers(self, username: str, xfer_ids: set) -> None:
+        """Cancel the given in-flight slskd transfers for a peer. Best-effort: a cleanup failure
+        must never mask the original download failure."""
+        for tid in xfer_ids:
+            try:
+                requests.delete(
+                    f"{self._url}/api/v0/transfers/downloads/{username}/{tid}",
+                    headers=self._headers(),
+                    timeout=15,
+                )
+            except Exception:
+                pass
+
     def _retrieve(self, files: list[dict], dest: str) -> int:
         """Move the just-finished slskd files out of its downloads dir into `dest`.