feat(memory-service): implement Paperclip Document Revisions, Diff Viewer, and Collaborative Annotation Threads

- Added database models and repositories for DocumentRevision, DocumentAnnotationThread, DocumentAnnotationComment.
- Added REST APIs for document revisions (/api/documents/{id}/revisions) and annotations (/api/documents/{id}/annotations, /api/annotations/{id}/resolve).
- Built GitHub-style Split-View Diff Viewer (LCS dynamic line diff algorithm) with Transitions.dev spring motion.
- Added inline text annotation bubbles and multi-agent Slide-over Discussion Drawer in Memory Hub.
- Integrated automated cross-agent review workflow (Architect PRD critique -> PM resolution -> Coder/QA notification).
- Verified with 810/810 passing tests (544 Pytest + 266 Bun frontend/E2E) and clean Vite build.