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

app/api/__init__.py

@@ -14,6 +14,7 @@ from app.api.azure import router as azure_router
 from app.api.backup import router as backup_router
 from app.api.billing import router as billing_router
 from app.api.classification_rules import router as classification_rules_router
+from app.api.comments import router as comments_router
 from app.api.compliance import router as compliance_router
 from app.api.database import router as database_router
 from app.api.diagnostic import router as diagnostic_router
@@ -108,3 +109,4 @@ router.include_router(system_reset_router)
 router.include_router(translation_router)
 router.include_router(classification_rules_router)
 router.include_router(automation_router)
+router.include_router(comments_router)