Build per-invoice activity export to JSON for archival

[Kingsman-99]

Description

AuditLogTable.tsx displays an invoice's audit log in the UI but offers no export for offline archival/compliance record-keeping. This issue adds a structured JSON export.

Technical Context

Involves src/components/AuditLogTable.tsx. Add an "Export JSON" button producing a complete structured record: invoice metadata, full audit log entries, and payment history, suitable for long-term archival outside the app.

Acceptance Criteria

• Export includes invoice core fields, full audit log (action, actor, timestamp), and payment list
• Output is valid, pretty-printed JSON with a documented schema version field
• Filename includes invoice ID and export timestamp: invoice-{id}-archive-{timestamp}.json
• Works correctly for invoices with empty audit logs (no payments/actions yet)
• Unit test: schema version field present, correct filename format, empty-log handling

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