fix: subtitle casing

[omgate234]

Pull Request

Description:
Fix Video.addSubtitle so subtitle styles actually reach the backend. The method was sending camelCase keys under
subtitle_style and had no defaults, so any caller config was silently ignored by the API and addSubtitle() with no args sent
subtitle_style: {}. Now it merges the SDK's existing SubtitleStyleDefaultValues and converts keys to snake_case before
posting, matching the HttpClient contract and the Python SDK's behavior.

Changes:

• Bugfix: convert subtitle_style payload from camelCase → snake_case via fromCamelToSnake in Video.addSubtitle
  (src/core/video.ts)
• Bugfix: apply SubtitleStyleDefaultValues when addSubtitle is called with no config or a partial config, so the request
  mirrors videodb-python's SubtitleStyle() dataclass defaults
• Imports: pull in fromCamelToSnake from @/utils and SubtitleStyleDefaultValues from @/core/config

Related Issues:

• Closes #

Testing:

• Manually invoked video.addSubtitle() (no args) and verified the outbound request body contains snake_case keys (font_name,
  primary_colour, border_style, margin_l, …) populated from SubtitleStyleDefaultValues, and that the call returns a valid
  streamUrl.
• Manually invoked video.addSubtitle({ fontName: "Inter", fontSize: 24, marginV: 40 }) and verified the partial overrides
  merge over the defaults and serialize as font_name, font_size, margin_v while the rest of the defaults are still sent.
  confirm CLI-supplied flags now actually take effect.
• npx tsc --noEmit passes.

Checklist:

• Code follows project coding standards
• Tests have been added or updated
• Code Review
• Manual test after merge
• All checks passed