Improve Django admin: search, aggregations, bulk actions, and Commitment visibility

[Fingolfin7]

Motivation

• The admin UX was missing searchable/filterable views and the Commitment model wasn’t visible for quick management, making high-volume admin tasks slow and error-prone.
• Operators need user-level aggregates and bulk operations to triage and update records efficiently.

Description

• Reworked core/admin.py to register Projects, SubProjects, Sessions, Context, Tag, and Commitment with rich ModelAdmin configurations including list_display, list_filter, search_fields, autocomplete_fields, and list_editable.
• Added bulk admin actions for projects (status transitions) and commitments (activate/deactivate, enable/disable banking) and exposed banking fields for quick edits.
• Added per-row aggregations using annotated querysets (counts and total minutes) for projects, subprojects, sessions, contexts, and tags to surface user-level context.
• Upgraded users/admin.py UserAdmin to display and annotate project_count, session_count, commitment_count, and total_logged_minutes, and added core/test_admin.py to validate admin registrations and the new user aggregation columns.

Testing

• Ran SECRET_KEY=test GEMINI_API_KEY=test AUDIT_PERIOD=60 python manage.py check which completed with no issues.
• Ran SECRET_KEY=test GEMINI_API_KEY=test AUDIT_PERIOD=60 python manage.py test core.test_admin which executed the admin tests (2 tests) and passed (OK).

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Codex Task