fix(backend): make packages/backend/tsconfig.json build under TypeScript 6

[drk-crew]

Goal

Goal: fix(backend): make packages/backend/tsconfig.json build under TypeScript 6

Why it matters

npm run build fails on main for the backend (CI run 37245569802, jobs build and docker-build;

What changed

packages/backend/tsconfig.json | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

Tests

• crewverify on 2b5479759279 in a clean checkout: backend ok, build-shared ok, check-035f3c97 ok, install ok
• inherited base failures (already on main, not caused here): 0
• verifier verdict: pass

Risk and provenance

• risk class: R1
• crew task: DRK-TSCONFIG-TS6, tier cheap
• declared paths: packages/backend/tsconfig.json, packages/backend/jest.config.js

AI assistance

• Written by an AI crew (Kiro CLI workers, Claude verifier); opened by the crew broker. A human reviews and merges.