deps: upgrade prisma 6.x → 7.x (major)

[JasonTM17]

Dependabot major bump: prisma + @prisma/client 6.19.3 → 7.8.0 (backend). Requires compat audit + ADR per global rule #29.

Scope

• backend ORM client + CLI
• Schema regeneration + migration test
• Generated types may shift breaking call sites

Action items

• Read Prisma 7 migration guide
• Run pnpm prisma generate on TS 7 client and run typecheck
• Test migrations apply cleanly on staging DB
• Update CI prisma generate step if API changed

Refs

• https://github.com/JasonTM17/foodflow/pull/closed (dependabot PR)
• ADR slot: docs/adr/0006-prisma-7-upgrade.md (TBD)