feat(contract): CT-12 BatchRevocationEndpoint & CT-13 DocumentExistenceCheckEndpoint

[Ibinola]

Summary

Implements two new contract endpoints

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CT-13 — DocumentExistenceCheckEndpoint (closes #545)

Route: GET /exists/:hash

• Returns { exists: bool, cached: bool } only — no transaction details
• Checks Redis cache first (same key used by /verify endpoint)
• Falls back to Stellar verify_hash on cache miss
• Caches positive results with no TTL (document existence is permanent)
• Caches negative results with a 60-second TTL
• Returns 400 for invalid hash format via HashValidator::validate_sha256
• Implementation: contract/src/module/exists/mod.rs

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CT-12 — BatchRevocationEndpoint (closes #544)

Route: POST /revoke/batch

• Accepts a JSON array of up to 20 RevokeRequest objects (document_hash, reason, revoked_by)
• Returns 400 if the array is empty or exceeds 20 items
• Processes all revocations concurrently using tokio::join_all
• Each item in the response contains: document_hash, success, tx_hash (on success), error (on failure)
• Partial success is allowed — a failed item does not abort others
• Duplicate / already-revoked hashes are reported as per-item errors
• Implementation: contract/src/module/batch_revoke/mod.rs

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Testing

• cargo build passes cleanly with no warnings or errors.

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