Implement Dashboard Search

Description:
Enable users to search dashboards by name. For the first iteration, limit to name-based search; future iterations can expand to description and config keys.

Acceptance Criteria:

• search_vector column added to dashboards table and indexed with GIN.
• Background worker to populate/update search_vector on create/update.
• GraphQL query dashboards(search: String) returns matching dashboards by name.
• Response times under 500ms for typical queries.
• Unit and integration tests for search functionality.