Replace `railpack_environment_variables_collection()` helper (which returned
pre-filtered Eloquent attribute collections) with inline queries on
`environment_variables()` / `environment_variables_preview()` filtered by
`is_buildtime`. This ensures Railpack build variables are sourced from the
same query path as the rest of the deployment pipeline and avoids relying on
a now-removed accessor that silently included all railpack vars regardless of
build context.
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
