fix(deploy): prevent frontend image tag swaps

[fj-sunny]

Build backend, user frontend, and Admin images sequentially, then validate the frontend image contents before replacing production containers.\n\nCloses #184

[fennoai]

Review conclusion

The production deployment change keeps image replacement gated behind three sequential service builds, and the added image-content checks correctly distinguish the user frontend from the Admin build using the Admin Vite base path. The explicit deploy-frontend:latest and deploy-admin:latest tags match the production Compose definitions, so the validation examines the same images later consumed by up --no-build.

Verified with bash -n, git diff --check, focused positive/negative fingerprint checks, and the passing deployment-script CI job. I found no actionable correctness, reliability, security, or maintainability regressions in the fixed diff.