Feat: video preview generation and subtitle support

[arabcoders]

This pull request introduces several enhancements and documentation updates related to external subtitle support, preview clip endpoints, and recommended chunk size handling, along with improvements to environment configuration and CI checks. The most significant changes are the addition of new API endpoints for subtitle discovery and preview clips, updates to API documentation, and expanded environment variable support for subtitles and previews.

API and Feature Documentation Updates:

• Added documentation for new API endpoints: /api/tokens/{download_token}/uploads/{upload_id}/subtitles (for listing external subtitle tracks), /api/tokens/{download_token}/uploads/{upload_id}/subtitles/{source_format} (for serving subtitle content), and /api/tokens/{download_token}/uploads/{upload_id}/preview.mp4 (for serving preview clips for embeds). These endpoints are now fully described in API.md, including authentication, parameters, responses, and error codes. [1] [2]
• Documented the new /api/metadata/extract endpoint, which extracts metadata from filenames based on regex patterns defined in the dynamic metadata schema. [1] [2]
• Updated API responses and documentation to include the recommended_chunk_bytes field for uploads, guiding clients on optimal TUS chunk size for resumable uploads. [1] [2] [3] [4] [5]

Environment and Configuration:

• Added new environment variables and config options for external subtitle support (FBC_SUBTITLE_PATH, FBC_SUBTITLE_CACHE_TTL_SECONDS) and for preview clip generation (FBC_EMBED_PREVIEW_CLIP_SECONDS, FBC_EMBED_PREVIEW_MIN_SIZE_BYTES). These are now validated and documented in README.md and backend/app/config.py. [1] [2] [3] [4] [5]
• Improved Dockerfile locale handling by installing and configuring en_US.UTF-8 to ensure consistent Unicode support. [1] [2]

CI/CD Improvements:

• Added a frontend format check (bun run format:check) to the build and main GitHub Actions workflows to enforce code style in pull requests. [1] [2]

These changes enhance the usability and robustness of the uploader, especially for users needing subtitle support and improved client guidance for resumable uploads.