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/__init__.py

@@ -2,7 +2,7 @@
 Grid package — restructured from grid_* flat modules.
 
 Backward-compatible re-exports: consumers can still use
-``from grid_build_core import ...`` etc. via the shim files in backend/.
+``from grid.build.core import ...`` etc. via the shim files in backend/.
 
 Sub-packages:
   - grid.build   — grid construction pipeline (_build_grid_core and phases)