commit_msg.txt

fix: add external anchoring to evidence blockchain, fix BSA 63(4) compliance

Closes #1220 - Evidence blockchain was single-DB linked list without external anchoring

- EvidenceRecord.java: added merkleRoot, externalAnchorProof, anchoredAt, anchorService fields
- EvidenceAnchoringService.java: Merkle tree computation + simulated external anchoring
- BlockchainEvidenceService: upload now computes Merkle root, anchors it externally,
  updates all case evidence records with proof; verifyEvidence and verifyChain both
  validate external anchor alongside block hash integrity
- CertificateService: BSA 63(4) certificates now include Merkle root, anchor service,
  and anchor proof in the PDF output
- BlockchainService: remains the SHA-256 engine; anchoring provides the external
  verifiability layer