 Implement Recurring Payment Subscriptions

**Type**: Feature  
**Priority**: High  
**Labels**: `feature`, `backend`, `subscriptions`

**Description**:
Add subscription management for recurring payments.

**Features**:
- Subscription creation
- Auto-renewal
- Cancellation
- Grace periods
- Failed payment handling

**Acceptance Criteria**:
- Subscriptions working
- Auto-renewal reliable
- Dunning management
- All CI/CD checks must pass

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