Implement fragment caching for notifications and enhance cache manage…

[rsmithlal]

…ment

• Added NotificationCacheManagement concern for automatic cache expiration on notification updates and deletions.
• Introduced multi-level fragment caching in notification views to optimize rendering performance.
• Updated notification partials to utilize fragment caching for specific content sections (header, content, footer).
• Created a dropdown content view for notifications with caching logic for empty states and individual notifications.
• Enhanced the notifications controller with a new dropdown action and caching strategy.
• Added tests for notification caching behavior, including cache warming and expiration logic.
• Documented the fragment caching implementation and its benefits in a new markdown file.
• Updated locale files to include translations for no notifications message.
• Created factories for noticed notifications to facilitate testing.

Summary

Describe the change and the motivation.

Checklist

• Tests added/updated and passing (bin/ci).
• Lint and security checks (rubocop, brakeman, bundler-audit).
• Documentation updated under docs/ describing new/changed functionality.
• Mermaid diagrams (docs/*.mmd) updated to reflect changes.
• Rendered PNGs regenerated with bin/render_diagrams and committed.
• For DB changes, included any needed backfills/dedupes and noted risks.

Screenshots / Diagrams

If applicable, include screenshots or link to updated diagrams.

Notes

Anything reviewers should be aware of (migration order, flags, feature toggles).