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/app/downloads/storage.py

@@ -181,16 +181,23 @@ def _nfo_xml(meta: MediaMeta) -> str:
     return "\n".join(parts)
 
 
+def _sidecar(media: Path, ext: str) -> Path:
+    """The sibling sidecar path (`.nfo` / `.jpg`) for a media file. Swaps the extension on the FULL
+    media path — NOT via `media.with_suffix("")` first: stripping the real extension leaves a stem
+    whose OWN last dot may be inside the title (e.g. "Sonka Házilag."), which `with_suffix` would
+    then read as the extension and clobber, orphaning the sidecar under a truncated name."""
+    return media.with_suffix(ext)
+
+
 def write_sidecars(
     download_root: str, rel: str, meta: MediaMeta, thumb_src: Path | None
 ) -> bool:
     """Write `<basename>.nfo` + `<basename>.jpg` next to the media file. Returns True if written."""
     media = abs_path(download_root, rel)
-    base = media.with_suffix("")
     try:
-        base.with_suffix(".nfo").write_text(_nfo_xml(meta), encoding="utf-8")
+        _sidecar(media, ".nfo").write_text(_nfo_xml(meta), encoding="utf-8")
         if thumb_src and thumb_src.exists():
-            shutil.copyfile(str(thumb_src), str(base.with_suffix(".jpg")))
+            shutil.copyfile(str(thumb_src), str(_sidecar(media, ".jpg")))
         return True
     except OSError:
         return False
@@ -199,8 +206,7 @@ def write_sidecars(
 def delete_asset_files(download_root: str, rel: str) -> None:
     """Remove the media file and its sidecars; prune now-empty parent dirs (best effort)."""
     media = abs_path(download_root, rel)
-    base = media.with_suffix("")
-    for p in (media, base.with_suffix(".nfo"), base.with_suffix(".jpg")):
+    for p in (media, _sidecar(media, ".nfo"), _sidecar(media, ".jpg")):
         try:
             p.unlink()
         except OSError: