deps: upgrade typescript 5.x → 6.x (major)

[JasonTM17]

Dependabot major bump: typescript 5.9.3 → 6.0.3 in backend + web. Requires compat audit + ADR per global rule #29 (deps triage). PR closed pending review.

Scope

• backend: tsc 5.x → 6.x compatibility
• web: shared monorepo workspaces (admin, restaurant, packages/ui, packages/i18n)

Action items

• Run pnpm --filter ... typecheck on each workspace with TS 6 to surface breaking changes
• Review TS 6 release notes for deprecated features in use
• Update tsconfig if needed
• Bump in single PR after green local typecheck

Refs

• https://github.com/JasonTM17/foodflow/pull/closed (dependabot PR)
• ADR slot: docs/adr/0005-typescript-6-upgrade.md (TBD)