feature: bulk tokenization via spreadsheet import

[nanaf6203-bit]

Summary

Developers fill the tokenise form one row at a time.

Affected area

• Provide CSV/Excel import with schema validation, preview, and a bulk-create endpoint.

Acceptance criteria

• Implement the change described above.
• Add or update unit/e2e tests locking in the new behaviour.
• Update README / docs to reflect the new state.

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Filed automatically as part of the PropChain code-review sweep.