Expanded the special discounts system to support:
- Discount types: percentage, fixed amount, or free subscription
- Date range (effective_from/effective_to) for time-bounded offers
- Conditions: none (direct), full_payment, or min_payment threshold
- Bonus: free subscription years granted when discount activates
- Applies-to targeting: membership_fee, subscription, or all

Added SpecialDiscountService for evaluating conditional discounts and
applying free subscription bonuses on member.activated event.

Updated form UI with dynamic visibility and validation.

Migration: Phase_98_003 adds new columns to special_discounts table.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>