Remove dead backend Dockerfiles and sync-lock script

Commit notes

Dockerfile.local can't build (copies bun.lock from backend/, which doesn't exist — the lockfile lives at repo root) and Dockerfile.distroless is unused and installs without a lockfile; production deploys via Railpack per railway.json, not any of these. Drop the sync-lock script that existed solely to manufacture a lockfile for Dockerfile.local, and update docs (DEPLOYMENT.md, AGENTS.md) that referenced the removed files/script.