Count the peers actually tried, not the attempt counter

A job carried over from before the candidate ladder has attempts=0 while
carrying a real failure in `tried`, so the summary rendered as "tried 0 peers:
1x no bytes in 20m" -- a count of zero next to the evidence contradicting it.
The live instance produced exactly that when the stall check finally cleared
its long-stuck job.

Counts distinct usernames in `tried` instead. The two agree for anything this
version enqueued; the list is the one that is right in both cases.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app.py

@@ -895,13 +895,20 @@ def no_candidate_detail(seen: int, stats: "collections.Counter",
 
 
 def exhausted_detail(tried: list[dict], attempts: int) -> str:
-    """Summarise the whole ladder instead of overwriting it with the last error."""
+    """Summarise the whole ladder instead of overwriting it with the last error.
+
+    Counts `tried`, not `attempts`. They agree for anything this version
+    enqueued, but a row carried over from before the ladder existed has
+    attempts=0 with a real failure recorded against it, which rendered as the
+    nonsense "tried 0 peers: 1x no bytes in 20m". The list is the evidence.
+    """
     if not tried:
         return "no candidate could be enqueued"
+    n_peers = len({t.get("username") for t in tried if t.get("username")}) or len(tried)
     reasons = collections.Counter(t["reason"].split(":")[0] for t in tried)
-    plural = "" if attempts == 1 else "s"
+    plural = "" if n_peers == 1 else "s"
     joined = ", ".join(f"{n}x {r}" for r, n in reasons.most_common(3))
-    return f"tried {attempts} peer{plural}: {joined}"
+    return f"tried {n_peers} peer{plural}: {joined}"
 
 
 async def try_next_candidate(job_id: str, artist: str, title: str,