feat: add 14-day and 21-day audit expiry experiment variants

[hiteshkava]

Summary

Extend the audit expiry experiment by introducing two additional variants:

• expiry_14_days

This allows testing the impact of longer audit content availability periods alongside the existing 7-day variant and control group.

Changes

Added Experiment Variants

• Added VARIANT_EXPIRY_14_DAYS
• Updated VALID_VARIANTS to include the new variants

Updated Expiry Calculation Logic

Enhanced compute_audit_expiry_at() to support:

• 7-day expiry
• 14-day expiry

The function now returns:

• Expiry timestamp based on variant configuration
• Corresponding expiry duration in days

Example

Variant	Expiry Calculation
control	Content availability date + access duration
expiry_7_days	Content availability date + 7 days
expiry_14_days	Content availability date + 14 days

Testing

• Verified new variants are accepted in VALID_VARIANTS
• Verified expiry dates are correctly calculated for 14-day variant
• Confirmed existing control and 7-day behavior remains unchanged

Jira: https://2u-internal.atlassian.net/browse/LP-981