Fix: Update all old-style imports inside packages to new paths

65 files in klausur-service packages + 3 in backend-lehrer packages
had stale imports referencing deleted shim modules.

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

klausur-service/backend/grid/editor/api.py

@@ -8,7 +8,7 @@ The actual endpoints live in:
   - grid_editor_api_unified.py (build-unified-grid, unified-grid)
 
 This module re-exports the combined router and key symbols so that
-existing `from grid_editor_api import router` / `from grid_editor_api import _build_grid_core`
+existing `from grid.editor.api import router` / `from grid.editor.api import _build_grid_core`
 continue to work unchanged.
 """
 
@@ -20,7 +20,7 @@ from .api_box import router as _box_router
 from .api_unified import router as _unified_router
 
 # Re-export _build_grid_core so callers that do
-# `from grid_editor_api import _build_grid_core` keep working.
+# `from grid.editor.api import _build_grid_core` keep working.
 from grid.build.core import _build_grid_core  # noqa: F401
 
 # Merge all sub-routers into one combined router