feat(comments): add document comments, annotations, and @mention support

- Add DocumentComment and DocumentAnnotation models to app/models.py
- Create migration 041_add_document_comments_and_annotations
- Add API endpoints for CRUD operations on comments and annotations
- Add threaded comment support with parent_id relationships
- Add @mention extraction from comment body text
- Add resolve/unresolve comment thread endpoint
- Add mentionable users endpoint (GET /api/users/mentionable)
- Add 43 unit tests covering all endpoints and edge cases
- Add 29 i18n translation keys to en.json
- Update API documentation in docs/API.md

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/DocuElevate/sessions/3894af37-0f19-457b-8811-f1feb18b17ef

frontend/translations/en.json

@@ -315,6 +315,20 @@
   "admin_users.total_count_users": "{count} users",
   "admin_users.total_no_users": "No users",
   "admin_users.total_one_user": "1 user",
+  "annotations.add": "Add annotation",
+  "annotations.color": "Color",
+  "annotations.content_placeholder": "Write an annotation...",
+  "annotations.delete_confirm": "Are you sure you want to delete this annotation?",
+  "annotations.deleted": "Annotation deleted",
+  "annotations.empty": "No annotations yet",
+  "annotations.heading": "Annotations",
+  "annotations.page": "Page",
+  "annotations.save": "Save",
+  "annotations.type_highlight": "Highlight",
+  "annotations.type_note": "Note",
+  "annotations.type_strikethrough": "Strikethrough",
+  "annotations.type_underline": "Underline",
+  "annotations.updated": "Annotation updated",
   "api_tokens.col_created": "Created",
   "api_tokens.col_expires": "Expires",
   "api_tokens.col_last_ip": "Last IP",
@@ -484,6 +498,21 @@
   "billing.success_heading": "You're all set!",
   "billing.success_message": "Your subscription has been activated. Thank you for choosing DocuElevate!",
   "billing.success_page_title": "DocuElevate - Subscription Activated",
+  "comments.add_comment": "Add comment",
+  "comments.add_reply": "Reply",
+  "comments.body_placeholder": "Write a comment... Use @username to mention someone",
+  "comments.delete_confirm": "Are you sure you want to delete this comment?",
+  "comments.deleted": "Comment deleted",
+  "comments.edit": "Edit",
+  "comments.empty": "No comments yet",
+  "comments.heading": "Comments",
+  "comments.mention_users": "Mention users",
+  "comments.reply_placeholder": "Write a reply...",
+  "comments.resolve": "Resolve",
+  "comments.resolved": "Resolved",
+  "comments.save": "Save",
+  "comments.unresolve": "Reopen",
+  "comments.updated": "Comment updated",
   "common.actions": "Actions",
   "common.active": "Active",
   "common.all": "All",