Add on-chain aggregate stats tracking (total invoices, total volume)

[Kingsman-99]

Overview

There is no on-chain source of truth for protocol-wide stats. Add instance-storage aggregate counters that track total invoices created, total XLM volume, and total recipients paid across all time.

Acceptance Criteria

• Instance storage fields: stats_total_invoices: u64, stats_total_volume: i128, stats_total_recipients_paid: u64
• Counters incremented atomically in create_invoice, pay_invoice, and release_funds respectively
• get_stats() view function returns all three counters
• Emits StatsUpdated { total_invoices, total_volume, total_recipients_paid, ledger } event on each change
• Counters overflow-safe: use checked_add with StatsOverflow error
• Stats are instance storage (persists without rent; no TTL)
• Integration tests: counters after 1 invoice cycle, counters after 5 invoices, overflow guard

[Adesam007-pr3dator]

@Adesam007-pr3dator has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi, I would love to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Adesam007-pr3dator to this issue.

[ndyugwu]

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

i would like to work on this

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ndyugwu to this issue.

[drips-wave]

Congratulations, @ndyugwu! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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