add withdrawal request API with penalty calculation

[Joewizy]

Closes #460

Summary

• Add POST /savings/withdraw endpoint with subscription ID, amount, and optional reason
• Calculate 5% early withdrawal penalty for locked (FIXED) products; no penalty for flexible or matured subscriptions
• Create WithdrawalRequest entity with full status tracking (PENDING → PROCESSING → COMPLETED/FAILED)
• Record withdrawals in transaction ledger, update subscription balance, and cancel subscription if fully withdrawn
• Send email + in-app notification on withdrawal completion via withdrawal.completed event

Test plan

• POST /savings/withdraw with valid subscription returns correct penalty/netAmount
• Early withdrawal from FIXED product applies 5% penalty
• Withdrawal from FLEXIBLE product has zero penalty
• Withdrawal from matured FIXED product has zero penalty
• Withdrawal exceeding balance returns 400
• Withdrawal from non-owned subscription returns 404
• Withdrawal from cancelled subscription returns 400
• Transaction ledger entry created with correct metadata
• Subscription amount decremented after withdrawal
• Email notification sent on completion

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Project	Deployment	Actions	Updated (UTC)
nestera	Ready	Preview, Comment	Mar 29, 2026 5:10pm

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