feat(workspace): add debounced search filters for captures and items

[hammond01]

Summary

• add q search parameter to captures/items list APIs
• implement case-insensitive content filtering in backend services
• add integration coverage for q filtering on captures and items
• add debounced search inputs in workspace UI:
  • capture inbox search
  • converted items search
• wire search through frontend API client query contracts

Validation

• backend: ./gradlew test
• frontend tests: npx vitest run
• frontend build: npm run build

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