Emit payment milestone events at 25%, 50%, 75%, and 100% funded

[Kingsman-99]

Overview

There are no events emitted when an invoice crosses a funding threshold. Add milestone events so off-chain indexers and the app can react to progress without polling get_invoice.

Acceptance Criteria

• After each pay_invoice, check if funded_amount crossed a milestone threshold since last check
• Milestones: 25%, 50%, 75%, 100% of target_amount
• Each milestone emitted at most once per invoice (tracked in instance storage)
• Event: MilestoneReached { invoice_id, milestone_bps: u32, funded_amount: i128, ledger: u32 }
• milestone_bps: 2500, 5000, 7500, 10000 for 25/50/75/100%
• Multiple milestones can be crossed in a single payment (e.g. one large payment crosses 50% and 75%) — emit both events
• Milestone tracking stored per invoice; does not affect rent cost significantly
• Integration tests: single payment crossing 2 milestones, exact milestone hit, overfunded invoice

[dunnidev]

@dunnidev has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I can fix this issue

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[Agbasimere]

@Agbasimere has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi there, I'd like to work on this issue.

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[stpatrickghost]

@stpatrickghost has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi Boss... I can handle this professionally and make sure that your issue is closed quickly without errors.

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[Happybello365]

@Happybello365 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello i would like to work on this issue

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