Add the application-layer helpers that decide which files are worth
parsing, sitting one notch above parse_release.
- is_supported_video(path, kb): extension-only check against
kb.video_extensions. Lowercased suffix lookup. Directories and
broken symlinks return False.
- find_main_video(folder, kb): top-level scan only (no recursion into
subdirectories — releases that wrap their video in Sample/ are
PATH_OF_PAIN territory). Lexicographically-first eligible file wins
when several qualify (deterministic, no size-based ranking). A bare
file as folder argument is supported for single-file releases.
No size threshold and no filename heuristics ('sample' / 'trailer'):
the parser's job is to extract structure, not to second-guess
non-standard release shapes. PoP catches the rest.
17 tests under tests/application/test_supported_media.py.
