feat: implement full RBAC role-based access control

Backend:
- deps.py: add require_admin, require_manager, require_labeler, require_guest_or_above
- user.py: all write endpoints require admin
- datasource.py: write/sync endpoints require admin
- metadata.py: sync endpoint requires admin
- classification.py: category/rule write requires admin; results query requires guest+ with data isolation
- project.py: GET requires manager with created_by filtering; DELETE checks ownership
- task.py: my-tasks requires labeler with assignee_id filtering; create-task requires manager
- dashboard.py: requires guest_or_above
- report.py: requires guest_or_above
- project_service: list_projects adds created_by filter; list_results adds project_ids filter

Frontend:
- stores/user.ts: add hasRole, hasAnyRole, isAdmin, isManager, isLabeler, isSuperadmin
- router/index.ts: add roles to route meta; beforeEach checks role permissions
- Layout.vue: filter menu routes by user roles
- System.vue: hide add/edit/delete buttons for non-admins
- DataSource.vue: hide add/edit/delete/sync buttons for non-admins
- Project.vue: hide add/delete buttons for non-admins

backend/app/api/v1/dashboard.py

@@ -9,7 +9,7 @@ from app.models.metadata import DataSource, DataTable, DataColumn
 from app.models.project import ClassificationResult, ClassificationProject
 from app.models.classification import Category, DataLevel
 from app.schemas.common import ResponseModel
-from app.api.deps import get_current_user
+from app.api.deps import get_current_user, require_guest_or_above, _is_admin
 
 router = APIRouter()
 
@@ -17,7 +17,7 @@ router = APIRouter()
 @router.get("/stats")
 def get_dashboard_stats(
     db: Session = Depends(get_db),
-    current_user: User = Depends(get_current_user),
+    current_user: User = Depends(require_guest_or_above),
 ):
     """Dashboard overview statistics based on real DB data."""
     data_sources = db.query(DataSource).count()
@@ -42,7 +42,7 @@ def get_dashboard_stats(
 @router.get("/distribution")
 def get_dashboard_distribution(
     db: Session = Depends(get_db),
-    current_user: User = Depends(get_current_user),
+    current_user: User = Depends(require_guest_or_above),
 ):
     """Distribution data for charts based on real DB data."""
     # Level distribution