docs: add collaboration to FEATURES, ARCHITECTURE, and OpenAPI spec

- Add note sharing section to FEATURES.md
- Document collaboration access control model in ARCHITECTURE.md
- Add collaborators and user search endpoints to openapi.yaml
- Regenerate API.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/FEATURES.md

@@ -80,6 +80,18 @@
 - Restore notes from trash
 - Permanently delete from trash
 
+### Note Sharing / Collaboration
+- Share notes with other users on the same instance
+- User search autocomplete for adding collaborators
+- Full collaboration: both owner and collaborators can edit content, color, and checklist mode
+- Per-user state: pin, archive, and sort order are independent per user
+- Owner-only actions: trash, delete, share/unshare
+- Collaborator indicator (Users icon) on shared note cards
+- Hover popover showing collaborator names
+- Share dialog accessible from editor toolbar
+- Collaborators can leave shared notes
+- Shared notes appear mixed in main list alongside owned notes
+
 ### Full-Text Search
 - Search across note titles, content, and tags
 - Real-time search results as you type