Fix Rails 8.1+ deployment by adding SECRET_KEY_BASE to release script

Rails 8.1.1 (introduced in commit baac683) requires SECRET_KEY_BASE
to be set during initialization, even for tasks like db:prepare that
don't actually use the secret key.

This change updates the release script to provide a placeholder
SECRET_KEY_BASE value if one isn't already set in the environment,
similar to the fix in commit 6039dd7 for the Docker build phase.

The actual SECRET_KEY_BASE for runtime will still come from the
deployment environment variables.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

.controlplane/release_script.sh

@@ -14,7 +14,9 @@ log 'Running release_script.sh per controlplane.yml'
 
 if [ -x ./bin/rails ]; then
     log 'Run DB migrations'
-    ./bin/rails db:prepare || error_exit "Failed to run DB migrations"
+    # SECRET_KEY_BASE is required for Rails 8.1+ initialization but not used for migrations
+    # The actual secret key will be provided at runtime by the environment
+    SECRET_KEY_BASE="${SECRET_KEY_BASE:-precompile_placeholder}" ./bin/rails db:prepare || error_exit "Failed to run DB migrations"
 else
     error_exit "./bin/rails does not exist or is not executable"
 fi