fix(downloads): keep sidecar basenames intact when the title contains a dot

write_sidecars / delete_asset_files computed `base = media.with_suffix("")` and then
`base.with_suffix(".nfo")`. Stripping the real `.mp4` first leaves a stem whose own last dot may
be inside the title (e.g. "Sonka Házilag."), which `with_suffix` then treats as the extension and
clobbers — writing the .nfo/.jpg under a TRUNCATED basename that no longer matches the media, so
Plex/Kodi never associate them (and delete_asset_files targets the wrong name, leaking orphans).
Swap the extension on the FULL media path via a shared `_sidecar` helper instead.

Pre-existing bug (predates the source-adapters work); found during its E2E and folded in here.
Verified end-to-end: a dotted-title Facebook reel now writes a single shared basename.

backend/tests/test_storage.py

@@ -147,3 +147,26 @@ def test_nfo_uniqueid_type_reflects_the_source():
     base = dict(video_id="v1", title="t", uploader="c", upload_date=None)
     assert '<uniqueid type="youtube"' in _nfo_xml(MediaMeta(**base))  # default preserved
     assert '<uniqueid type="facebook"' in _nfo_xml(MediaMeta(**base, uniqueid_type="facebook"))
+
+
+def test_sidecars_keep_the_full_basename_when_the_title_has_a_dot(tmp_path: Path):
+    # A title ending in a period ("Sonka Házilag.") puts a dot in the media slug; the .nfo/.jpg
+    # sidecars must sit on the SAME (full) basename, not be truncated at that dot (which orphaned
+    # them from the media so Plex/Kodi never matched them).
+    from app.downloads.storage import write_sidecars
+
+    stem = "Chan_-_2026-08-12_-_Sonka_Hazilag._Villam_konyha_abc_a5"
+    rel = f"Chan/Season_2026/{stem}.mp4"
+    media = tmp_path / rel
+    media.parent.mkdir(parents=True)
+    media.write_text("x")
+    thumb = tmp_path / "t.jpg"
+    thumb.write_bytes(b"\xff\xd8\xff")
+    meta = MediaMeta(video_id="abc", title="Sonka Házilag. | Villám konyha", uploader="Chan",
+                     upload_date=None)
+
+    assert write_sidecars(str(tmp_path), rel, meta, thumb) is True
+    names = sorted(p.name for p in media.parent.iterdir())
+    assert f"{stem}.nfo" in names, names
+    assert f"{stem}.jpg" in names, names
+    assert "Chan_-_2026-08-12_-_Sonka_Hazilag.nfo" not in names  # the old truncated orphan