refactor(domain): strip live filesystem I/O from VOs and entities

DDD-pure cleanup — entities and value objects no longer query the world
at read time.

  FilePath: drop .exists() / .is_file() / .is_dir(). The VO is now a
    pure address; ask the injected FilesystemScanner for live state.
  Movie:    drop .has_file() / .is_downloaded(). Invariant: when the
    application sets file_path, it has already constated the file
    exists; downstream readers trust the snapshot.
  Episode:  same — drop .has_file() / .is_downloaded().
  SubtitlePlacer: drop the pre-check .exists() calls. The placer now
    attempts os.link() and reports FileNotFoundError / FileExistsError
    as skip reasons. Removes a TOCTOU race as a bonus.

Tests adjusted: the FilePath VO method tests are gone (the methods are
gone), test_has_file_false_when_no_path replaced by a plain assertion
on file_path is None. Placer tests are unchanged — the skip-reason
strings ('not found', 'already exists') match the new try/except paths.

The 'snapshot value objects' pattern (ProbedMediaInfo, TmdbMovieInfo)
that this cleanup enables is documented in refactor_domain_io.md, to
be applied when a future use case actually needs richer metadata —
not now, no speculative VOs.

tests/domain/test_tv_shows.py

@@ -157,10 +157,9 @@ class TestEpisode:
         assert filename.startswith("S01E05")
         assert "Gray.Matter" in filename
 
-    def test_has_file_false_when_no_path(self):
+    def test_file_path_unset_by_default(self):
         e = self._ep()
-        assert not e.has_file()
-        assert not e.is_downloaded()
+        assert e.file_path is None
 
     def test_str_format(self):
         e = self._ep(season=2, episode=3, title="Bit by a Dead Bee")