feat(subtitles): add regex subtitle filters with built-in presets (#318)

- Add `subtitleFilter` config with four presets: parentheses, brackets, speaker prefix and music symbols. Custom regex rules run after the presets.
- Filter primary text where it enters the app (live `sub-text` and parsed cues), so the overlay, sidebar, mining and stats all get the same filtered text.
- Changes hot-reload: the current line is read again and the active track is parsed again, using the filter config as the prefetch parse key.
- Add a custom rule editor in Settings under Behavior → Subtitle Filters that checks patterns as you type.
- Update the config example, the configuration docs and the architecture domains doc, and add a changelog fragment.

changes/subtitle-filters.md

@@ -0,0 +1,5 @@
+type: added
+area: subtitles
+
+- Added subtitle filters (`subtitleFilter`) that remove unwanted text from primary subtitles before it reaches the overlay, sidebar, mined sentences and stats. Built-in presets cover text in parentheses (speaker names, readings, notes; whole-line inner thoughts are kept), square-bracket sound descriptions, speaker names before a colon, and music notes. Custom regex rules with a replacement can be added after the presets. Lines left with no words are hidden.
+- Configure them in the settings window under Behavior → Subtitle Filters; changes apply live to the current subtitle file.