🧹 Code Health Improvement: Remove duplicate method updateVideoCompleted

[LimaniBhavik]

🎯 What: The code health issue addressed
Removed the duplicated method updateVideoCompleted from lib/Utils/DatabaseHelper.dart.

💡 Why: How this improves maintainability
The method updateVideoCompleted was an exact duplicate of updateVideo and had no existing callers in the entire codebase. Removing it cleans up dead code, reduces cognitive load, and eliminates unnecessary duplication.

✅ Verification: How you confirmed the change is safe

1. Analyzed the code and found updateVideoCompleted matches updateVideo verbatim.
2. Used grep -rn "updateVideoCompleted" . to confirm that the method is never called in lib/ or test/.
3. Ran relevant unit tests (flutter test test/video_model_test.dart) to ensure no existing functionality is broken.
4. Passed formatting and code review successfully.

✨ Result: The improvement achieved
Reduced duplication and improved codebase cleanliness without altering any existing behavior.

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PR created automatically by Jules for task 9448472922760440816 started by @LimaniBhavik

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