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)

backend/tests/test_naming.py

@@ -0,0 +1,63 @@
+"""ASCII-slug naming (no spaces, no accents) for on-disk paths and served filenames, and the
+social engagement-prefix strip in title normalization."""
+from app.downloads import storage
+from app.titles import normalize_title
+
+
+def _is_ascii_slug(s: str) -> bool:
+    return s.isascii() and " " not in s
+
+
+def test_sanitize_folds_accents_to_ascii():
+    assert storage.sanitize("Barnabás Skriván") == "Barnabas_Skrivan"
+    assert storage.sanitize("Comedy Central Magyarország") == "Comedy_Central_Magyarorszag"
+    assert storage.sanitize("Marco Mobili Bútoráruház Veszprém") == "Marco_Mobili_Butoraruhaz_Veszprem"
+    # Hungarian double-acute (ő/ű) and German ß.
+    assert storage.sanitize("Előző Műsor") == "Elozo_Musor"
+    assert storage.sanitize("Straße") == "Strasse"
+
+
+def test_sanitize_output_is_pure_ascii_without_spaces():
+    for name in ("Poénbomba", "Mágáné Zsuzsanna", "café déjà vu", "naïve œuvre"):
+        out = storage.sanitize(name)
+        assert _is_ascii_slug(out), out
+
+
+def test_sanitize_non_latin_falls_back_not_crashes():
+    # A title with no ASCII base collapses to the untitled fallback rather than an empty name.
+    assert storage.sanitize("日本語") == "untitled"
+
+
+def test_display_filename_uses_underscores_and_ascii():
+    out = storage.display_filename("Amikor a férfi éhesen ér haza")
+    assert out == "Amikor_a_ferfi_ehesen_er_haza"
+    assert _is_ascii_slug(out)
+
+
+def test_download_filename_appends_single_extension():
+    from pathlib import Path
+
+    assert storage.download_filename("Szép Videó", "mp4", Path("x.mp4")) == "Szep_Video.mp4"
+
+
+def test_rel_path_is_ascii_everywhere():
+    meta = storage.MediaMeta(
+        video_id="abc123", title="Áristom Ödön", uploader="Hajdú Balázs", upload_date=None
+    )
+    rel = storage.rel_path(meta, "mp4", "plex", asset_id=7)
+    assert _is_ascii_slug(rel.replace("/", ""))
+    assert "Hajdu_Balazs" in rel and "Aristom_Odon" in rel
+
+
+def test_engagement_prefix_stripped():
+    assert normalize_title(
+        "1.6M views · 66K reactions | Amikor a férfi éhesen ér haza"
+    ) == "Amikor a férfi éhesen ér haza"
+    assert normalize_title("13M views · 162K reactions |Valami cím") == "Valami cím"
+    assert normalize_title("500 views | Rövid") == "Rövid"
+
+
+def test_engagement_prefix_does_not_touch_normal_titles():
+    # A pipe that isn't an engagement prefix must survive.
+    assert normalize_title("Vlog 12 | A nagy nap") == "Vlog 12 | A nagy nap"
+    assert normalize_title("How I built a boat") == "How I built a boat"

backend/tests/test_storage.py

@@ -88,7 +88,8 @@ class TestRelPath:
 
 class TestDownloadFilename:
     def test_appends_the_container_extension(self):
-        assert download_filename("My Video", "mp4", Path("/x/f.webm")) == "My Video.mp4"
+        # Served names follow the same ASCII-slug policy as on-disk names: spaces → underscores.
+        assert download_filename("My Video", "mp4", Path("/x/f.webm")) == "My_Video.mp4"
 
     def test_does_not_double_an_extension_already_present(self):
         assert download_filename("clip.mp4", "mp4", Path("/x/f.mp4")) == "clip.mp4"