[codex] Add OpenAI-compatible subtitle translation fallback

[rasariogualco-crypto]

Summary

• Adds a plain chat-completions fallback when structured OpenAI subtitle translation streaming fails.
• Parses JSON returned by OpenAI-compatible providers, including fenced JSON blocks.
• Covers the parser and fallback path with focused Vitest tests.

Why

Some OpenAI-compatible providers, such as DeepSeek-style endpoints, can pass model connectivity tests but fail DashPlayer subtitle translation because structured streaming may not produce output. This keeps the existing structured path and falls back to a more compatible JSON text response.

Validation

• ./node_modules/.bin/vitest run src/backend/application/services/impl/tests/openAiSubtitleBatchParser.test.ts src/backend/application/services/impl/tests/TranslateServiceImpl.openaiFallback.test.ts