[BE-121] Unit tests for the verification module

[mftee]

Overview

backend/src/verification/ has 3 files, 1 entity, and no tests. Verification links a document to its on-chain proof, so a bug here produces a false claim of verification — actively worse than no verification at all.

Tasks

• Test verification record creation and retrieval.
• Test that verification of an unanchored document returns a clear negative rather than a false positive.
• Test hash-mismatch handling: a document whose content no longer matches its anchored hash must be reported as failed verification, not as an error.
• Test idempotency — verifying twice must not create duplicate records.
• Test the failure path when the Stellar layer is unreachable: verification should report "unknown", never "verified".
• Target ≥80% coverage.

Acceptance Criteria

• A test proves an unanchored or mismatched document never reports as verified.
• Stellar unavailability yields an explicit indeterminate state.
• npm run test -- verification passes at ≥80% coverage.

Notes for Contributors

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