fix: detect platform on video create, remove hardcoded twitter default

backend/app/services/downloader.py

@@ -55,6 +55,17 @@ def _is_pornhub_url(url: str) -> bool:
     return bool(PORNHUB_URL_RE.match(url))
 
 
+def detect_platform(url: str) -> str:
+    """Detect platform from URL."""
+    if _is_twitter_url(url):
+        return "twitter"
+    if _is_youtube_url(url):
+        return "youtube"
+    if _is_pornhub_url(url):
+        return "pornhub"
+    return "unknown"
+
+
 def _is_twitter_url(url: str) -> bool:
     return bool(TWITTER_URL_RE.match(url))
 
@@ -556,5 +567,5 @@ def download_video(url: str, format_id: str = "best", progress_callback=None) ->
         "filename": os.path.basename(filename),
         "file_path": filename,
         "file_size": file_size,
-        "platform": "twitter",
+        "platform": detect_platform(url),
     }