feat: add upload analytics API with key metrics and usage insights

[Krushna-Pisal]

This PR adds a new analytics endpoint to provide useful insights into the upload system. It helps in understanding usage patterns, tracking performance, and identifying failures more easily.

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###What is Added

A new endpoint:

GET /api/upload/stats

This endpoint returns key metrics such as:

• Total uploads
• Successful uploads
• Failed uploads
• Success rate (%)
• Average upload time (in seconds)
• Uploads in the last 24 hours

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###Additional Insights

To make the API more useful for monitoring and future UI integration, the following are also included:

• Status distribution (success / failed / processing)
• File type distribution (e.g., jpg, pdf, png)
• Daily upload trend (last 7 days)

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###Implementation Notes

• Uses SQLAlchemy aggregation queries for better performance
• Handles edge cases like empty datasets and null values
• Follows a modular structure with a separate service layer
• Keeps response format consistent with existing APIs

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Sample Response

{
  "status": "success",
  "data": {
    "total_uploads": 250,
    "successful_uploads": 210,
    "failed_uploads": 40,
    "success_rate": 84.0,
    "average_upload_time": 3.1,
    "uploads_last_24h": 22,
    "status_distribution": {
      "success": 210,
      "failed": 40,
      "processing": 5
    },
    "file_type_distribution": {
      "jpg": 80,
      "pdf": 60
    },
    "daily_upload_trend": [
      {"date": "2026-04-20", "count": 10},
      {"date": "2026-04-21", "count": 15}
    ]
  }
}

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🚀 Why This Matters

This adds basic observability to the upload system and can be useful for:

• Monitoring system health
• Debugging upload failures
• Supporting future dashboard or admin UI

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Future Improvements

• Add filters (date range, file type, user)
• Integrate with frontend dashboard
• Export analytics data