docs: add public sharing to features, architecture, and API docs

- FEATURES.md: add public link sharing feature details
- ARCHITECTURE.md: add shared_notes table to schema section
- openapi.yaml: add share create/revoke, public note, and public
  attachment endpoints with SharedNoteData schema
- Regenerate API.md

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/FEATURES.md

@@ -81,13 +81,20 @@
 - Permanently delete from trash
 
 ### Note Sharing / Collaboration
+- **Public link sharing**: share any note via a public URL — no login required for viewers
+  - Toggle public link on/off from the share dialog
+  - Copy shareable URL with one click
+  - Read-only public page with full markdown/checklist/image rendering
+  - Public links are automatically invalidated when a note is trashed
+  - Rate-limited public endpoints (60 req/min per IP)
+  - Globe icon on note cards indicates a public link is active
 - 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
+- Sharing indicator on note cards (Globe for public, Users for collaborators-only)
+- Hover popover showing sharing status and collaborator names
 - Share dialog accessible from editor toolbar
 - Collaborators can leave shared notes
 - Shared notes appear mixed in main list alongside owned notes