fix: Improve CSV badge awarding with validation and error reporting

[Marvinrose]

Fix: CSV badge awarding with validation and error reporting

Fixes #348

Problem

The award_from_csv endpoint existed to allow admins to award badges in bulk via CSV upload, but had a TODO requesting validation and feedback that was never implemented.

The original code had file validation issues, bytes read as strings, silent failures with no admin feedback..

Fix

To fix this issue, all i did was:

• Validate the file is present and has a filename before processing
• Decode each line from bytes to string explicitly using UTF-8
• Normalise column order: both email,badge_id and badge_id,email are accepted
• Validate badge existence before attempting to award
• Auto-create users who don't exist, preserving original behaviour and report which users were created
• Track and report already-awarded badges instead of silently skipping
• Flash a complete summary after processing, covering all outcomes