fix: keep generated filenames within the filesystem limit (closes #1034)

Sites that put a description in the title produce output names past what
the filesystem accepts, and the download fails outright with
"[Errno 36] File name too long". The name is now shortened to fit inside
prepare_filename, which every output path already passes through, so the
main file, its chapter files, thumbnails and subtitles stay consistent.

The limit is read from the filesystem (PC_NAME_MAX, falling back to 255)
and counted in bytes, not characters: a title of accented or CJK
characters reaches it in a third of the characters. The extension is
kept, a cut landing inside a multi-byte character does not leave a broken
sequence, and a reserve is held back for the suffixes yt-dlp appends
afterwards -- '.part', '.ytdl', '.f<format_id>', '-Frag<n>' -- since it
is those that pushed the reported name over the limit.

Names that already fit are untouched, so nothing that downloads today
changes name.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/tests/test_ytdl_utils.py

@@ -376,6 +376,49 @@ class ConfinedYoutubeDLTests(unittest.TestCase):
         self.assertEqual(self._prepared_path(""), "")
         self.assertEqual(self._prepared_path("-"), "-")
 
+    def test_overlong_name_is_trimmed_to_fit_the_filesystem(self):
+        # A title long enough to blow the filename limit is what made these
+        # downloads fail outright with [Errno 36] File name too long.
+        long_path = os.path.join(self.base, "a" * 400 + ".mp4")
+
+        result = self._prepared_path(long_path)
+
+        name = os.path.basename(result)
+        self.assertTrue(name.endswith(".mp4"))
+        self.assertLessEqual(len(name.encode("utf-8")), 255 - 32)
+        self.assertEqual(os.path.dirname(result), self.base)
+        # The file must still be writable once yt-dlp adds its own suffixes.
+        self.assertLessEqual(len(f"{name}.f1229065279304024v.part".encode("utf-8")), 255)
+
+    def test_name_within_the_limit_is_left_alone(self):
+        ok = os.path.join(self.base, "Ordinary Title.mp4")
+        self.assertEqual(self._prepared_path(ok), ok)
+
+    def test_limit_counts_bytes_not_characters(self):
+        # 200 CJK characters are 600 bytes: a character count would pass this.
+        long_path = os.path.join(self.base, "音" * 200 + ".mp4")
+
+        name = os.path.basename(self._prepared_path(long_path))
+
+        self.assertLessEqual(len(name.encode("utf-8")), 255 - 32)
+        # A trim landing mid-character must not leave a broken byte sequence.
+        self.assertEqual(name, name.encode("utf-8").decode("utf-8"))
+        self.assertTrue(name.endswith(".mp4"))
+
+    def test_a_long_tail_is_not_mistaken_for_an_extension(self):
+        # os.path.splitext on a title containing a dot late in the string would
+        # otherwise "preserve" a 100-character extension and trim nothing.
+        long_path = os.path.join(self.base, "b" * 300 + "." + "c" * 100)
+
+        name = os.path.basename(self._prepared_path(long_path))
+
+        self.assertLessEqual(len(name.encode("utf-8")), 255 - 32)
+
+    def test_trimming_still_cannot_escape_the_download_directory(self):
+        escaping = os.path.join(self.base, "..", "..", "d" * 400 + ".mp4")
+        with self.assertRaises(ytdl.yt_dlp.utils.DownloadError):
+            self._prepared_path(escaping)
+
 
 class SanitizeEntryForPickleTests(unittest.TestCase):
     def test_nested(self):