Implement feature gates for gradual feature rollout with kill switch

[Smartdevs17]

Description

New features are deployed to all users at once. Implement feature gates with gradual rollout and emergency kill switch.

Acceptance Criteria

• Feature flag configuration management
• Gradual rollout percentages (10%, 25%, 50%, 100%)
• User segment targeting for flags
• A/B test assignment and tracking
• Kill switch for immediate feature disable
• Feature flag analytics and stale flag detection

Technical Scope

• src/services/featureFlags.ts
• src/config/features.ts
• backend/services/
• Edge: flag evaluation performance, flag configuration conflicts, flag cleanup