Add modality field for efficient content filtering

[rtibbles]

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Overview

Currently, the modality value is stored in the ContentNode options JSONField, which makes filtering operations very inefficient. This tracking issue covers the work to add a dedicated modality field for efficient querying and to resolve related filtering issues.

Implementation approach

Add a modality CharField to the ContentNode model (in models.py as a locally-calculated cached field) with a database index. This field will be populated during content import by extracting the value from options['modality'].

Technical details

• Field type: CharField(max_length=50, blank=True, null=True, db_index=True, choices=modalities.choices)
• Location: kolibri/core/content/models.py (local field, not exported by Studio)
• Annotation: Extract from options['modality'] during channel import (similar to bitmask fields)
• Upgrade task: Backfill existing channels
• Expected values: QUIZ, SURVEY, CUSTOM_NAVIGATION, and future additions (5-15 total values anticipated)

🤖 This issue was written by Claude Code, under supervision, review and final edits by @rtibbles 🤖