Implement subscription dunning with progressive escalation

[Smartdevs17]

Context

Dunning needs progressive escalation.

Current Limitation

Dunning is basic with no escalation.

Expected Outcome

• Progressive dunning escalation
• Escalation rules
• Dunning analytics
• Dunning optimization

Acceptance Criteria

• Progressive dunning escalation
• Configurable escalation rules
• Dunning analytics
• Dunning optimization
• Dunning templates
• Dunning API
• Dunning documentation