Remove Docker from CI integration tests

[jakebromberg]

Replace Docker image builds and container orchestration in the CI Integration-Tests job with GitHub Actions service containers (PostgreSQL) and native Node.js processes (auth + backend).

Problem

The Integration-Tests job averages ~7 minutes, but only ~7 seconds is actual test execution. The rest is Docker overhead: building 3 images, starting 4 containers via Docker Compose, and cleaning up. The ECR caching strategy never hits for auth/backend because shared/ changes on nearly every PR.

Solution

• PostgreSQL: Move from Docker Compose to a GHA services: block (zero build cost)
• db-init: Run node dev_env/init-db.mjs directly on the runner
• Auth + Backend: Start as background node processes after npm run build
• Remove: Buildx setup, ECR login, image pull/build logic, merge-base SHA tracking, Docker Compose up/down

Estimated savings

Metric	Before	After
Integration-Tests job	~7 min	~2 min
Docker image builds	3 images (~4 min)	0
Container orchestration	~2.5 min	~15s

What stays the same

• detect-changes logic for whether to run integration tests
• npm ci and node_modules caching
• Jest test execution (npm run ci:test)
• Local CI scripts (ci-env.sh, ci-test.sh) -- Docker-based path unchanged for local dev

Supersedes

• ci: skip redundant Docker builds in CI #158 (skip redundant Docker builds) -- moot if Docker is removed
• ci: fix Docker builder-stage layer caching #159 (fix Docker layer caching) -- moot if Docker is removed

See Proposal 3 for full analysis and data.