Restructure: Move grid_* + vocab_* into packages (klausur-service)

grid/ package (16 files):
  grid/build/   — core, zones, cleanup, text_ops, cell_ops, finalize
  grid/editor/  — api, helpers, columns, filters, headers, zones

vocab/ package (10 files):
  vocab/worksheet/ — api, models, extraction, generation, ocr, upload, analysis, compare
  vocab/           — session_store, learn_bridge

26 backward-compat shims. Internal imports relative. RAG untouched.

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

klausur-service/backend/grid/__init__.py

@@ -0,0 +1,10 @@
+"""
+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/.
+
+Sub-packages:
+  - grid.build   — grid construction pipeline (_build_grid_core and phases)
+  - grid.editor  — FastAPI endpoints, helper functions, column/zone logic
+"""