v2.8.9 — Fix PHP 8.2 dynamic property warnings

• Declare $secret_key, $public_key, $payment_identifier, $product_code, $testmode explicitly
• Fixes PHP 8.2 deprecated dynamic property warnings on production
• No behaviour changes

v2.8.8

v2.8.8 - Fix nullable return type and price display interval only

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

v2.8.7

v2.8.7 - Fix price display: show billing interval only, hide total_pa…

v2.8.6

v2.8.6 - Style billing period suffix on single course page

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

v2.8.5

v2.8.5 - Fix single course page price suffix via correct Tutor LMS hook

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

v2.8.4

v2.8.4 - Fix single course page price via get_tutor_course_price filter

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

v2.8.3

v2.8.3 - Fix double price suffix and single course page price display

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

v2.8.2

v2.8.2 - Fix price display: show 100 Rwf / 6 months on course pages

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

v2.8.1

v2.8.1 - Switch cron back to production timing (daily/twicedaily)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

v2.8.0

v2.8.0 - Total payments / installment system with permanent course ow…