feat: Implement initial /report/generate endpoint

[olindost]

Overview: This PR introduces the initial POST /report/generate endpoint to kick off report generation.

Changes

• Added a new POST /report/generate endpoint in app/api/v1/routes.py.
• Endpoint logic is delegated to a new app/services/report_service.py for better separation of concerns.
• The endpoint immediately returns a report_id and a processing status, using an in-memory placeholder for now.
• Includes a basic ReportRequest model for input validation.

[felixjordandev]

Nice, this'll make starting report generation way easier! 🎉 Approved.