feat: add FP attachment library — attach existing CVE documents to FP submissions

- Add GET /api/ivanti/fp-workflow/documents/search endpoint for querying the document library
- Update POST /api/ivanti/fp-workflow to accept libraryDocIds for attaching library documents on create
- Update POST .../submissions/:id/attachments to accept libraryDocIds on edit
- Add AttachmentSourcePicker component with local upload and library search modes
- Integrate picker into FpWorkflowModal (create) and FpEditModal (edit)
- Track attachment source (local/library) in attachment_results_json for traceability

.kiro/specs/fp-attachment-library/.config.kiro

@@ -0,0 +1 @@
+{"specId": "acff93bd-0045-4fcd-b948-cc52c7cc5ec6", "workflowType": "requirements-first", "specType": "feature"}