refactor(server): reduce Sonar complexity

[aksOps]

Summary

• add deterministic regression coverage for board JSON PUT CAS and forge/AI preview contracts
• replace 12 duplicated server literals with byte-identical constants
• reduce cognitive complexity in 10 production functions across board handlers, AI coercion, and forge import/integration paths
• add focused error-path tests required by the 95% new-code coverage gate
• preserve HTTP, revision, idempotency, token isolation, pagination, provenance, and current all-invalid preview behavior

Verification

• 418 server tests passed; 418 server race tests passed
• GitHub Actions full Go race, vet, formatting, frontend coverage, Go coverage, and build gates passed
• Go coverage: 97.8% total; server package 98.3%
• 905 frontend tests passed; build transformed 197 modules
• staticcheck, gopls, complexity, duplication, parameter-count, and diff checks passed
• all 18 commits have valid ED25519 signatures
• independent lane, coverage, and integrated reviews: APPROVE

Sonar

• quality gate: OK at exact head 13a0dfffa773aa8bac326b755a2323f8bd48019e
• open PR issues: 0
• new coverage: 99.3%
• new duplicated lines: 0.0%
• resolves 12 go:S1192 and 10 go:S3776 production findings

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
99.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud