PM-6126: apply forum database migrations before startup

[jmgasper]

Summary

• run Prisma migrations before starting Forums API v6
• include the Prisma schema, migration history, configuration, and CLI in the production image
• rebuild the approved Prisma schema engine after script-free production dependency installation, so startup does not depend on a runtime download

Root cause

The reactions rollout added the PostReaction table and made topic detail reads query it, but the production image only included the generated Prisma client and started NestJS directly. A deployment whose database had not been migrated could therefore create and list topics successfully while every topic detail request failed, matching PM-6126.

The next deployment will apply the pending reactions migration before accepting traffic. Any future migration failure will stop the container instead of leaving the API partially available against an older schema.

Validation

• pnpm lint
• pnpm build
• sh -n appStartUp.sh
• production Docker image builds successfully
• production image contains the PM-6126-dependent reactions migration and an executable Alpine Prisma schema engine
• container startup invokes migrations and fails before NestJS when the database is intentionally unreachable
• full Jest run: 121 passed, 3 skipped; the two failures in forums-moderation.integration.spec.ts reproduce unchanged on origin/develop and are unrelated to this deployment-only change