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>

docker-compose.yml

@@ -8,7 +8,7 @@ services:
     environment:
       POSTGRES_DB: capakraken
       POSTGRES_USER: capakraken
-      POSTGRES_PASSWORD: capakraken_dev
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:?set POSTGRES_PASSWORD in .env (any non-empty value for local dev)}
     command: >
       postgres
       -c log_connections=on
@@ -61,7 +61,7 @@ services:
       # Always use the Docker-internal service name. The host-level DATABASE_URL
       # (localhost:5433) must not bleed into the container where "localhost" is
       # the container itself, not the host.
-      DATABASE_URL: postgresql://capakraken:capakraken_dev@postgres:5432/capakraken
+      DATABASE_URL: postgresql://capakraken:${POSTGRES_PASSWORD:?set POSTGRES_PASSWORD}@postgres:5432/capakraken
       REDIS_URL: redis://redis:6379
       NEXTAUTH_URL: ${NEXTAUTH_URL:?NEXTAUTH_URL must be set (e.g. https://your-domain.com)}
       NEXTAUTH_SECRET: ${NEXTAUTH_SECRET:?set NEXTAUTH_SECRET}