docs: expand token flow specification

[Kkkakania]

Closes #2

Summary

• Expand specs/token-flow.md into a formal end-to-end token lifecycle specification.
• Document all five required phases: eligibility ingestion, token issuance, token redemption, lost token reissue, and token invalidation.
• Add the required happy path, lost-token reissue, and failed-redemption sequence diagrams.
• Specify the exact vote redemption database transaction boundary and logging/audit constraints.

Verification

• Cross-checked core route and service names against AnonVote/core.
• Cross-checked crypto primitive names against AnonVote/js.
• Cross-checked Soroban helper and contract call names against AnonVote/core.
• Ran git diff --check.

Notes

• This repository is Markdown-only and has no package/build/test script to run.