Allow configuring the engine storage service independently

[akodkod]

When an application uses DB storage only for selected attachments, the engine still downloads and uploads through ActiveStorage::Blob.service. With Disk or S3 as the application default, DB attachment requests therefore reach the wrong service.

Add an optional engine configuration:

config.active_storage.service = :local
config.active_storage_db.service = :db

The controller resolves the configured string or symbol through the Active Storage service registry for both downloads and direct uploads. The default is nil, preserving the existing application-default behavior. Unknown names raise the registry error. The application default, signed URLs, and database schema are unchanged.

Includes mixed-storage documentation and request coverage for string/symbol names, downloads, direct uploads, fallback, and unknown names.

Validation

• Ruby 4.0.7 / Rails 8.1.4 / SQLite: 71 examples, 0 failures.
• The regression spec produces 5 failures out of 6 examples with the original upstream controller, and passes with this change.
• RuboCop: all changed Ruby files pass. The full run reports one existing Style/RedundantParentheses offense at spec/service/active_storage/service/db_service_spec.rb:195, unchanged from upstream main.
• Fasterer: passes.

For the full local suite, installed libvips and added ruby-vips ~> 2.3 through an ignored supplemental validation Gemfile; the currently resolved image_processing 2.2.0 does not bring that dependency into the standard bundle. No dependency changes are included in this PR. The existing upstream workflows cover the broader Rails/database matrix.