[Issue]: Add full-text search across activity description, subject, causer, and tags

[MrAdder]

Issue Type

Feature request

Summary

Add a searchable query interface that helps users find relevant activity records faster.

Problem / Context

Filters are useful when users know what they are looking for, but investigations often start with partial context. A broader search experience would make the activity resource more discoverable and useful.

Proposed Solution

Add a full-text or multi-field search capability that searches key activity fields such as description, subject, causer, and tags.

Acceptance Criteria

• Users can search across multiple high-value activity fields.
• Search results integrate cleanly with existing filters and sorting.
• Search behavior is documented and predictable.

Implementation Notes

Consider database compatibility, performance, and how search should interact with the existing table/filter setup.

Testing Notes

Add feature tests that verify relevant records are returned for representative search terms.

Additional Context

This is likely one of the highest-value review UX additions for 1.x.