feat: VOSTFR/VO/FRENCH detection and subtitle embedding

Query Arte Player API before each download to determine available stream
versions. Select lang tag (VOSTFR > VO, FRENCH if audio is fr). Embed
French subtitles as default MKV track when VOSTFR. All output now .mkv.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

arte_api.py

@@ -278,6 +278,35 @@ async def fetch_concerts(page: int = 1, search: str = "", page_size: int = 24, c
     }
 
 
+def get_versions(pid: str) -> list[dict]:
+    """Fetch available stream versions from Arte Player API for a programme ID."""
+    try:
+        raw = _fetch_url(
+            PLAYER_API.format(pid=pid),
+            headers={"User-Agent": _HEADERS["User-Agent"], "Accept": "application/json"},
+        )
+        data = json.loads(raw)
+        streams = data["data"]["attributes"].get("streams") or []
+        return streams[0].get("versions") or [] if streams else []
+    except Exception as ex:
+        logger.debug("Failed to get versions for %s: %s", pid, ex)
+        return []
+
+
+def select_lang_tag(versions: list[dict]) -> str:
+    """
+    Determine UNFR language tag from stream versions.
+    FR audio → FRENCH, non-FR + FR subs → VOSTFR, otherwise → VO.
+    """
+    if not versions:
+        return "VO"
+    if any(v.get("audioLanguage") == "fr" for v in versions):
+        return "FRENCH"
+    if any(v.get("subtitleLanguage") == "fr" for v in versions):
+        return "VOSTFR"
+    return "VO"
+
+
 async def invalidate_cache() -> int:
     _cache["ts"] = 0
     try: