feat(downloads): ASCII-slug names, universal browser playability, force re-download

Mini-epic "Downloads correctness & UX" (S1–S3), on top of the iOS-AV1 + 720p fixes.

S1 — naming & titles:
- ASCII-fold on-disk dir/file names AND the served (Content-Disposition) filename: no
  spaces (underscore-joined), no accents (á→a, ő→o, ß→ss), pure ASCII — portable and free
  of the `?` mojibake accented names show in non-UTF-8 tools. (Forward-only; existing files
  clean up via re-download.)
- Strip a leading social engagement prefix ("1.6M views · 66K reactions | …") from titles.

S2 — universal browser playability (generalizes the iOS re-encode):
- ensure_browser_playable guarantees the stored file is H.264 + AAC/MP3 in mp4 — the only
  combo every browser decodes. Re-encode just the offending stream(s) (video for AV1/VP9/HEVC,
  audio for Opus/etc), or remux when only the container is wrong. Gated on the mp4 compat
  profile (an explicit vcodec / non-mp4 custom profile opts out).

S3 — force re-download (preserves share links):
- POST /downloads/{job_id}/redownload keeps the job row so its public DownloadLink survives
  and resolves to the freshly-downloaded file; deletes the old file and re-fetches under the
  current rules. A failed re-download leaves the job in error (the intended broken-link case).
- Library "Re-download" action (icon + confirm), api method, hu/en strings.

Tests: naming/title-strip (test_naming), browser_transcode_flags matrix; existing
download_filename expectation updated for the underscore policy.

backend/tests/test_formats.py

@@ -49,11 +49,18 @@ def test_audio_only_unaffected():
     assert _opts({"mode": "a", "audio_format": "m4a"})["format"] == "bestaudio/best"
 
 
-def test_needs_ios_transcode_flags_only_undecodable_codecs():
-    assert editmod.needs_ios_transcode("av1") is True
-    assert editmod.needs_ios_transcode("vp9") is True
-    assert editmod.needs_ios_transcode("h264") is False
-    assert editmod.needs_ios_transcode("hevc") is False
-    # An unknown/failed probe must NOT trigger a needless re-encode.
-    assert editmod.needs_ios_transcode(None) is False
-    assert editmod.needs_ios_transcode("none") is False
+def test_browser_transcode_flags_universal_playability():
+    F = editmod.browser_transcode_flags
+    # Already universal: H.264 + AAC in mp4 → no change.
+    assert F("h264", "aac", True) == (False, False, False)
+    assert F("h264", "mp3", True) == (False, False, False)
+    # Non-H.264 video → full video re-encode (AV1/VP9 fail iOS; HEVC fails Chrome/FF).
+    assert F("av1", "aac", True) == (True, True, False)
+    assert F("vp9", "opus", True) == (True, True, True)
+    assert F("hevc", "aac", True) == (True, True, False)
+    # H.264 but bad audio → audio-only transcode (video copied).
+    assert F("h264", "opus", True) == (True, False, True)
+    # Codecs fine but wrong container → remux only (no re-encode).
+    assert F("h264", "aac", False) == (True, False, False)
+    # A failed probe / missing stream is never re-encoded.
+    assert F(None, None, True) == (False, False, False)