feat(railpack): add buildpack control var filtering and dev seeder

Extract NIXPACKS_/RAILPACK_ prefix filtering into a reusable
`scopeWithoutBuildpackControlVariables` query scope on EnvironmentVariable.
Apply scope consistently to runtime vars, runtime preview vars, and
buildtime var generation in ApplicationDeploymentJob.

Refactor `generate_railpack_env_variables` to return a Collection.
Add `RAILPACK_FRONTEND_IMAGE` constant and bake it into the
coolify-helper Dockerfile as a build arg.

Add DevelopmentRailpackExamplesSeeder (dev/local env only) for
seeding example Railpack apps, wired into DatabaseSeeder.

Add tests:
- ApplicationDeploymentControlVarFilteringTest: verifies control vars
  are excluded from runtime and buildtime envs
- DevelopmentRailpackExamplesSeederTest: verifies seeder behavior
- ApplicationDeploymentRailpackEnvParityTest: parity checks for env
  handling across build/runtime paths

docker/coolify-helper/Dockerfile

@@ -34,6 +34,8 @@ ARG MISE_VERSION
 
 USER root
 WORKDIR /artifacts
+ENV RAILPACK_VERSION=${RAILPACK_VERSION}
+ENV RAILPACK_FRONTEND_IMAGE=ghcr.io/railwayapp/railpack-frontend:v${RAILPACK_VERSION}
 RUN apk upgrade --no-cache && \
     apk add --no-cache bash curl git git-lfs openssh-client tar tini
 RUN mkdir -p ~/.docker/cli-plugins