feat: enforce a citation integrity gateway

[DivyamTalwar]

Summary

• parse the Standard/Deep verifier ledger with a strict Rust schema before completion can succeed
• require every verified claim to carry a credential-free HTTPS URL, substantive evidence quote, bounded confidence, and valid date
• reject unsafe optional URLs, distinguish source-specific receipts, and persist accepted receipts transactionally in SQLite
• make the README explicit that this validates evidence metadata but does not independently prove source content

Verification

• Rust CI runs clippy with warnings denied and the complete Rust test suite
• tests cover verified-claim gates, weak evidence handling, unsafe URL rejection, source-specific receipt identity, and idempotent SQLite persistence
• frontend lint/build remain part of the same CI workflow

This PR is stacked on #67. It must not merge until #67 is green and merged; once that happens, its diff against main reduces to the citation gateway.