fix: stop migrate image from building the frontend

[m3hrdadfi]

Bug

`docker compose up --build` intermittently failed with:

target migrate: failed to solve: process "/bin/sh -c npm ci" did not complete successfully: exit code: 139

Root cause

The migrate service built from the same Dockerfile/target as proxy, which meant building migrate also compiled the entire React frontend (node:22-alpine → npm ci → npm run build) — even though migrate only runs Alembic migrations and never touches the compiled frontend.

docker compose up --build builds migrate, proxy, and supabase-studio-gateway concurrently while pulling 7 base images at the same time. Under that combined load, npm ci running inside node:22-alpine (musl libc) segfaulted (exit 139). Confirmed this is a resource-contention flake, not a broken lockfile: the identical npm ci succeeds cleanly every time when run standalone, outside the concurrent build.

Fix

Split the Dockerfile into an app-runtime stage (Python venv + app code + migrations, no Node.js at all) and a proxy stage that extends it with the compiled frontend. migrate now targets app-runtime directly, so it never triggers the Node/npm build — removing an entire unnecessary, resource-heavy stage from the concurrent build graph.

Verified: docker compose build migrate now shows zero Node/npm steps, docker compose build proxy still ships frontend/dist correctly, and docker compose up -d brings the full stack up healthy end-to-end.

Test plan

• docker compose build migrate — no frontend-builder steps
• docker compose build proxy — frontend assets present in image
• docker compose up -d — all 8 services healthy, /health returns ok