Redesign Qt ImageCache invalidation strategy

This page is a capture in the next bucket of the product backlog — a pre-sprint idea, not yet pulled into a sprint as a story.

Note: this story should only be looked at after we move towards workspaces.

The current ImageCache invalidation approach is brittle and error-prone. When datasets are published, the cache reload is triggered by pattern-matching dataset codes (e.g., checking for "flag", "icon", "currenc", "countr" in the code string). This is fragile because:

• New dataset types require updating the pattern matching logic
• The relationship between datasets and cached data is implicit
• Cache invalidation relies on string matching rather than proper metadata

Proposed improvements:

• Use artefact_type metadata instead of code string matching to determine if cache invalidation is needed
• Consider a more general cache invalidation framework that can be extended to other caches (ChangeReasonCache, etc.)
• Investigate using server-side notifications to trigger cache invalidation (similar to the notification system already in place)
• Document the caching strategy and invalidation rules

Acceptance criteria:

• Cache invalidation is based on structured metadata, not string patterns
• Adding new image-related datasets doesn't require code changes
• Cache invalidation logic is documented and maintainable