fix: add Windows O_BINARY guard for binary file reads

Native Windows opens files in text mode by default, which corrupts
binary media files via CRLF translation. O_BINARY prevents this.

This completes the portability story — the PR was already correct on
all supported platforms (Linux/macOS/WSL2), but the round-3 change was
explicitly about portability, so we close the Windows path too.

api/routes.py

@@ -18424,7 +18424,12 @@ def _parse_range_header(range_header: str, file_size: int) -> tuple[int, int] |
 
 def _open_file_read_fd(target: Path, anchor_root: Path | None = None) -> int:
     if anchor_root is None:
-        return os.open(str(target), os.O_RDONLY)
+        flags = os.O_RDONLY
+        # On Windows, files are opened in text mode by default; O_BINARY
+        # prevents CRLF translation that would corrupt binary media files.
+        if hasattr(os, "O_BINARY"):
+            flags |= os.O_BINARY
+        return os.open(str(target), flags)
     return open_anchored_fd(anchor_root, target.resolve(), want_dir=False)