security(docker): remove hardcoded dev password, stop placeholder secrets leaking into migrator image (#50)

- docker-compose.yml: require ${POSTGRES_PASSWORD} for the postgres service
  and the app container's DATABASE_URL. No default — compose refuses to start
  without it, mirroring the existing PGADMIN_PASSWORD pattern.
- Dockerfile.prod: move auth/db ENV assignments from persistent ENV lines into
  an inline env prefix on the `pnpm build` RUN step. Placeholders are still
  available to `next build` but no longer persist in the builder layer or in
  the published migrator image (which is FROM builder).
- Dockerfile.dev: add HEALTHCHECK against /api/health and install curl for it.
- .dockerignore: cover nested **/.env*, **/*.pem, **/*.key, **/secrets/**.
- runtime-env.ts: add the CI build placeholder strings to the disallowed-secret
  set so a misconfigured prod deploy using the baked-in ARG defaults fails
  startup instead of silently running with a known-bad secret.
- .env.example: document the new POSTGRES_PASSWORD requirement.
- CI: write POSTGRES_PASSWORD into the Fresh-Linux Docker Deploy job's .env
  (must match docker-compose.ci.yml's hardcoded DATABASE_URL), and provide a
  dummy value in the E2E job where compose validates all services' interp.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Dockerfile.dev

@@ -1,7 +1,7 @@
 FROM node:20-bookworm-slim AS base
 
-# Prisma needs OpenSSL available during install/generate/runtime.
-RUN apt-get update -y && apt-get install -y openssl postgresql-client && rm -rf /var/lib/apt/lists/*
+# Prisma needs OpenSSL; curl is used by HEALTHCHECK below.
+RUN apt-get update -y && apt-get install -y openssl postgresql-client curl && rm -rf /var/lib/apt/lists/*
 
 # Install pnpm
 RUN npm install -g pnpm@9.14.2
@@ -30,4 +30,7 @@ RUN pnpm --filter @capakraken/db db:generate
 
 EXPOSE 3100
 
+HEALTHCHECK --interval=30s --timeout=5s --start-period=60s --retries=3 \
+  CMD curl -fsS http://localhost:3100/api/health || exit 1
+
 CMD ["sh", "./tooling/docker/app-dev-start.sh"]