Check the audio's length, since the filename can agree and still be wrong

The file that got filed as Michael Jackson's "Chicago" had every textual signal
in its favour: the title matched exactly, and once approved it landed under the
right artist, album and track number. The only thing that disagreed was the
running time -- 359 seconds against Deezer's 245 -- because it was The Fall.

Duration is the one signal a stranger's filename cannot fake, and muse already
had it: Deezer supplies it at request time and it has been sitting unused in the
jobs table. On import the real length is read off the downloaded audio and
compared; a drift past MUSE_LENGTH_TOLERANCE multiplies confidence down and
explains itself in the review queue rather than leaving a bare number.

For this case that is 0.75 -> 0.26, which falls below REVIEW_CONFIDENCE and so
reads as "almost certainly wrong" instead of "worth a look". It runs on import
rather than in confidence() because the file does not exist when the candidate
is chosen.

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

tests/test_acquisition.py

@@ -8,6 +8,7 @@ the server filters -- can be exercised deterministically.
 Run standalone: python tests/test_acquisition.py
 """
 import asyncio, json, os, sys
+from pathlib import Path
 
 sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 os.environ.setdefault("MUSE_AUTH", "off")
@@ -311,6 +312,36 @@ def main() -> int:
     print("\nsearch hygiene")
     check("finished searches are cleaned up", len(flaky.deleted) >= 1)
 
+    print("\nlength cross-check")
+    # The signal a filename cannot fake. Every textual cue agreed on the file
+    # that got filed as Michael Jackson; only the running time disagreed.
+    class FakeInfo:
+        def __init__(self, n): self.length = n
+
+    class FakeAudio:
+        def __init__(self, n): self.info = FakeInfo(n)
+
+    real_file = mutagen_file = app.mutagen.File
+    try:
+        app.mutagen.File = lambda p: FakeAudio(359.0)      # The Fall
+        got = app.length_check(Path("x.flac"), 245 * 1000)  # Michael Jackson
+        check("a 47% length disagreement is caught", got is not None, got)
+        check("and it drops the file below the review floor",
+              got and round(0.75 * got[0], 3) < app.REVIEW_CONFIDENCE,
+              got and round(0.75 * got[0], 3))
+        check("the reason names both lengths",
+              got and "5:59" in got[1] and "4:05" in got[1], got and got[1])
+
+        app.mutagen.File = lambda p: FakeAudio(247.0)
+        check("a two-second drift is not flagged",
+              app.length_check(Path("x.flac"), 245 * 1000) is None)
+
+        app.mutagen.File = lambda p: FakeAudio(359.0)
+        check("nothing to compare against is not a failure",
+              app.length_check(Path("x.flac"), None) is None)
+    finally:
+        app.mutagen.File = mutagen_file
+
     print("\nstall clock")
     from datetime import datetime, timedelta, timezone
     with app.db() as c: