Report incomplete scanned exam groups

[akarki2005]

Proposed Changes

Currently the exam scan log only surfaces per-page errors after scanning a batch of exam PDFs. There's no way to tell, at a glance, which student groups had all their pages successfully matched versus which are still missing pages.

This PR adds a per-group completion status to the scan log:

• Backend (exam_templates_controller.rb): while building the page-level error data, also aggregate pages by group_id and mark a group complete if any of its pages has a status containing 'Saved to complete directory'. This is returned as a new group_data array alongside the existing page_data.
• Frontend (exam_scan_log_table.jsx): add a new ExamScanGroupsTable sub-component, rendered under a "Papers found" heading next to the existing "Pending errors" table, showing each group's name and a Complete/Incomplete status.

Fixes #6511.

Type of Change

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Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
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🚦 Test update (change that only adds or modifies tests)
📦 Dependency update (change that updates a dependency)
📖 Documentation update (change that updates documentation)
🔧 Internal (change that only affects developers or continuous integration)

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