Add system design: wallet, balances, and ledger

[gardusig]

Goal

Specify wallet / stored value: double-entry ledger, concurrency, audit trail, and regulatory-safe boundaries. This note lives in gardusig/interviewing as interview prep, not production runbooks.

Acceptance criteria

• New markdown under system-design/concepts/ or system-design/architecture/ (pick the closer fit) and linked from the relevant folder README.md index when appropriate.
• Covers: requirements, scale or traffic assumptions, core APIs or events, data model sketch, and failure modes or retries.
• Optional mermaid diagram if it clarifies data or control flow.
• Stays shareable: no confidential employer internals, customer PII, or unreleasable numbers—use rounded hypothetical loads.

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Source issue title (before rename)

Wallet / Balance System / payment

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References

• (none yet — add papers, docs, or diagrams when drafting.)