chore: introduced modular subtitler design

[jeduardo]

Introduce a factory to instantiate subtitlers for different providers and refactor the Gemini-based subtitler as a provider implementation. This facilitates the implementation of #31 and #32

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.67%. Comparing base (b2eb0db) to head (25627c5).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
- Coverage   97.79%   97.67%   -0.12%     
==========================================
  Files           5        9       +4     
  Lines         453      473      +20     
  Branches       63       64       +1     
==========================================
+ Hits          443      462      +19     
  Misses          2        2              
- Partials        8        9       +1     

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve positional constructor compatibility in factory

Switching AISubtitler from a dataclass constructor to a factory changed the first positional parameter from model_name to provider, so existing calls like AISubtitler("gemini-2.5-flash", "<api-key>") now interpret the model name as a provider and fail with Unsupported AI subtitle provider. This is a runtime regression for any programmatic users who instantiated AISubtitler positionally before this refactor.

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