feat: add invoice line-item metadata

[damiedee96]

Summary

Implements support for arbitrary key/value metadata on invoice line items.

Changes

• Added optional string metadata to InvoiceLineItem.
• Added validation for metadata shape, entry count, key length, and value length.
• Preserved metadata through invoice creation, updates, persistence, and retrieval.
• Rendered line-item metadata in printable invoice HTML and branded invoice output.
• Added metadata storage, validation, retrieval, and PDF rendering to the Soroban invoice contract.
• Added coverage for metadata validation and printable invoice rendering.

Testing

• Invoice print and metadata tests pass.
• Backend invoice customization test suite passes.
• Existing unrelated TypeScript errors remain in the repository.

Closes #1734

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