refactor: tech debt mini-pass (items 5, 6, 7, 20)

Low-risk cleanup items, no functional change to the parser. The
philosophy remains: keep the parser simple, the AI handles edge cases.

- Extract duplicated 'fs-safe title → dot-folder-name' regex into
  to_dot_folder_name() in domain/shared/value_objects.py. Used by both
  MovieTitle.normalized() and TVShow.get_folder_name() (item #5).
- ParsedRelease.languages now uses field(default_factory=list) instead
  of a manual __post_init__ assigning [] via object.__setattr__ (#6).
- tv_shows/entities.py module docstring: prepend ASCII ownership tree
  for quicker visual scan of the aggregate hierarchy (#7).
- file_extensions.yaml: split subtitle sidecars (.srt/.sub/.idx/.ass/.ssa)
  into a dedicated 'subtitle:' category instead of lumping them under
  'metadata:'. _METADATA_EXTENSIONS at the value_objects.py level remains
  the union of both — detect_media_type behavior unchanged. New loader
  load_subtitle_extensions() exposes the distinct subtitle set for future
  callers in the subtitles domain (#20).

Suite: 1020 passed, 8 skipped.

CHANGELOG.md

@@ -112,6 +112,22 @@ callers).
 
 ### Internal
 
+- **`to_dot_folder_name(title)` helper** in
+  `alfred/domain/shared/value_objects.py` — extracts the
+  `re.sub(r"[^\w\s\.\-]", "", title).replace(" ", ".")` pattern that was
+  duplicated between `MovieTitle.normalized()` and `TVShow.get_folder_name()`.
+- **`ParsedRelease.languages` uses `field(default_factory=list)`** instead of
+  a manual `__post_init__` that assigned `[]` via `object.__setattr__`.
+- **`file_extensions.yaml` splits subtitle sidecars (`.srt`, `.sub`, `.idx`,
+  `.ass`, `.ssa`) into a dedicated `subtitle:` category** instead of lumping
+  them under `metadata:`. The `_METADATA_EXTENSIONS` set used by
+  `detect_media_type` remains the union of both (same behavior — subtitles
+  are still ignored when deciding the media type of a folder), but a new
+  `load_subtitle_extensions()` loader is now available for the subtitles
+  domain. Sematic clarity, no functional change.
+- **`tv_shows/entities.py` module docstring** now shows the aggregate
+  ownership as an ASCII tree before the rule text — quicker visual scan
+  of the DDD structure.
 - Removed backward-compat shims `_sanitise_for_fs` /
   `_strip_episode_from_normalised` from `domain/release/value_objects.py`
   (zero callers).