feat(release): pre-pipeline exclusion helpers

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.

tests/application/test_supported_media.py

@@ -0,0 +1,130 @@
+"""Tests for the pre-pipeline exclusion helpers (Phase A bis)."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+from alfred.application.release.supported_media import (
+    find_main_video,
+    is_supported_video,
+)
+from alfred.infrastructure.knowledge.release_kb import YamlReleaseKnowledge
+
+_KB = YamlReleaseKnowledge()
+
+
+# --------------------------------------------------------------------- #
+# is_supported_video                                                    #
+# --------------------------------------------------------------------- #
+
+
+class TestIsSupportedVideo:
+    def test_mkv_is_supported(self, tmp_path: Path) -> None:
+        f = tmp_path / "movie.mkv"
+        f.touch()
+        assert is_supported_video(f, _KB) is True
+
+    def test_mp4_is_supported(self, tmp_path: Path) -> None:
+        f = tmp_path / "movie.mp4"
+        f.touch()
+        assert is_supported_video(f, _KB) is True
+
+    def test_uppercase_extension_is_supported(self, tmp_path: Path) -> None:
+        # File systems can return mixed case; we lowercase the suffix.
+        f = tmp_path / "movie.MKV"
+        f.touch()
+        assert is_supported_video(f, _KB) is True
+
+    def test_srt_is_not_video(self, tmp_path: Path) -> None:
+        f = tmp_path / "movie.srt"
+        f.touch()
+        assert is_supported_video(f, _KB) is False
+
+    def test_nfo_is_not_video(self, tmp_path: Path) -> None:
+        f = tmp_path / "movie.nfo"
+        f.touch()
+        assert is_supported_video(f, _KB) is False
+
+    def test_no_extension_is_not_video(self, tmp_path: Path) -> None:
+        f = tmp_path / "README"
+        f.touch()
+        assert is_supported_video(f, _KB) is False
+
+    def test_directory_is_not_video(self, tmp_path: Path) -> None:
+        d = tmp_path / "subdir.mkv"  # even with a video extension
+        d.mkdir()
+        assert is_supported_video(d, _KB) is False
+
+    def test_nonexistent_path_is_not_video(self, tmp_path: Path) -> None:
+        assert is_supported_video(tmp_path / "ghost.mkv", _KB) is False
+
+
+# --------------------------------------------------------------------- #
+# find_main_video                                                       #
+# --------------------------------------------------------------------- #
+
+
+class TestFindMainVideo:
+    def test_single_video_file_in_folder(self, tmp_path: Path) -> None:
+        main = tmp_path / "Movie.2020.mkv"
+        main.touch()
+        assert find_main_video(tmp_path, _KB) == main
+
+    def test_returns_lexicographically_first_among_multiple(
+        self, tmp_path: Path
+    ) -> None:
+        # Legitimate for season packs: pick the first episode by name.
+        ep2 = tmp_path / "Show.S01E02.mkv"
+        ep1 = tmp_path / "Show.S01E01.mkv"
+        ep2.touch()
+        ep1.touch()
+        assert find_main_video(tmp_path, _KB) == ep1
+
+    def test_skips_non_video_files(self, tmp_path: Path) -> None:
+        # nfo and srt come alphabetically before .mkv, must not win.
+        (tmp_path / "Movie.nfo").touch()
+        (tmp_path / "Movie.srt").touch()
+        vid = tmp_path / "Movie.mkv"
+        vid.touch()
+        assert find_main_video(tmp_path, _KB) == vid
+
+    def test_ignores_subdirectories(self, tmp_path: Path) -> None:
+        # A Sample/ subdir must NOT be descended into.
+        sample_dir = tmp_path / "Sample"
+        sample_dir.mkdir()
+        (sample_dir / "sample.mkv").touch()
+        main = tmp_path / "Movie.mkv"
+        main.touch()
+        assert find_main_video(tmp_path, _KB) == main
+
+    def test_only_subdirectory_with_video_returns_none(
+        self, tmp_path: Path
+    ) -> None:
+        # No top-level video, only one inside a subdir → None.
+        sub = tmp_path / "Sample"
+        sub.mkdir()
+        (sub / "video.mkv").touch()
+        assert find_main_video(tmp_path, _KB) is None
+
+    def test_empty_folder_returns_none(self, tmp_path: Path) -> None:
+        assert find_main_video(tmp_path, _KB) is None
+
+    def test_nonexistent_folder_returns_none(self, tmp_path: Path) -> None:
+        assert find_main_video(tmp_path / "ghost", _KB) is None
+
+    def test_single_file_release_passed_as_folder_arg(
+        self, tmp_path: Path
+    ) -> None:
+        # Some releases are a bare .mkv with no enclosing folder.
+        f = tmp_path / "Movie.2020.1080p.mkv"
+        f.touch()
+        assert find_main_video(f, _KB) == f
+
+    def test_single_file_non_video_passed_as_folder_arg(
+        self, tmp_path: Path
+    ) -> None:
+        f = tmp_path / "README.nfo"
+        f.touch()
+        assert find_main_video(f, _KB) is None