feat: Build automated dunning management with configurable retry strategies

[agenes01]

Fixes #706

Context

Failed subscription payments need automated dunning with configurable retry strategies.

Changes Made

• Updated src/types/dunning.ts to include FailureReason and RetryStrategy types.
• Enhanced IDunningService and DunningService in backend/services/billing/ to support configurable retry strategies per failure reason, template management, and A/B testing.
• Improved dunning analytics tracking to handle recovery rate and time to recovery accurately by tracking recovered entries.
• Modified src/store/subscriptionStore.ts and src/store/dunningStore.ts to utilize the new failure reason parameter when recording payment attempts or failures, integrating mock dunning flow with specific reasons.
• Configured progressive dunning (retry -> notification -> escalation) across backend logic.

Acceptance Criteria Handled

• Configurable retry strategies (insufficient funds, expired card, network)
• Progressive dunning (retry -> notification -> escalation)
• Dunning analytics (recovery rate, time to recovery)
• Dunning template management
• Dunning A/B testing
• Dunning reporting dashboard
• Dunning integration with payment providers (handled via interfaces and billing service implementations)

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