Improve curated results handling with per-collection flag

Problem/Motivation

Currently, the system does not identify which collections have curations configured.
As a result, it is not possible to know for which collections the parameter
enable_overrides should be enabled.
This means that curations can only be activated globally for all collections,
or must be manually handled by the frontend, which reduces flexibility and maintainability.

Steps to reproduce

1. Perform a search with multiple collections.
2. Notice that there is no information available about which collections have curations set.
3. Without this knowledge, it is not possible to selectively enable
   enable_overrides only where needed.

Proposed resolution

Introduce a way to flag collections that have curations enabled.
For example, add a has_curations parameter to
collection_render_parameters.
This parameter would then be available in Twig templates,
allowing the frontend to implement different UX behaviors based on whether
curations are available for a specific collection.
Additionally, the system could automatically set enable_overrides
only for those collections where has_curations is true.

Remaining tasks

• Add a mechanism to detect if a collection has curations configured.
• Expose this information as has_curations in
  collection_render_parameters.
• Update logic to set enable_overrides only where needed.
• Document usage and provide examples for Twig template customization.
• Test that curated results are applied only on collections with curations.

User interface changes

No direct UI changes, but Twig templates will have access to
has_curations to implement different UX if desired.

API changes

A new optional parameter has_curations would be introduced in
collection_render_parameters.
This change is backward-compatible, since the parameter is optional.

Data model changes

None.