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>

.dockerignore

@@ -17,11 +17,21 @@ node_modules
 *.swp
 *.swo
 
-# Environment files (injected at runtime)
+# Environment files (injected at runtime). Glob variants catch nested
+# .env, .env.local, etc. inside any package directory.
 .env
 .env.*
+**/.env
+**/.env.*
 !.env.example
 
+# Private keys, certificates, and any secrets-like directory. Defence in
+# depth against accidentally bind-mounting or COPYing these in.
+**/*.pem
+**/*.key
+**/secrets
+**/secrets/**
+
 # Test artifacts
 coverage
 **/coverage