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

database/seeders/DatabaseSeeder.php

@@ -31,5 +31,11 @@ class DatabaseSeeder extends Seeder
             CaSslCertSeeder::class,
             PersonalAccessTokenSeeder::class,
         ]);
+
+        if (in_array(config('app.env'), ['local', 'development', 'dev'], true)) {
+            $this->call([
+                DevelopmentRailpackExamplesSeeder::class,
+            ]);
+        }
     }
 }