Backend - Multi-file search and document generation

[SiriChandanaGarimella]

Is your feature request related to a problem? Please describe.
The backend currently doesn't support searching multiple files and combining results into a single output.

Describe the solution you'd like
Implement backend functionality to:

1. Handle multiple file search requests.
2. Combine search results from multiple files into a single Word document.
3. Modify the preview API to support multiple file results.

Describe alternatives you've considered

• Processing files sequentially instead of in parallel.
• Returning separate results for each file.

Acceptance Criteria

1. The backend API accepts and processes requests containing multiple file paths.
2. Search functionality is applied to each file independently using the provided search criteria.
3. Results from all files are combined into a single Word document, with clear separation and labeling for each file's results.
4. The preview API returns a structured response that includes results from all processed files.
5. Error handling is implemented to manage cases where some files fail to process while others succeed.
6. Ensure reasonable processing time for multiple files (specific time constraints to be determined based on expected use case).