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/editor/__init__.py

@@ -0,0 +1,15 @@
+"""
+Grid Editor sub-package — FastAPI endpoints and helper functions.
+
+Modules:
+  - api          — barrel re-export (combined router + _build_grid_core)
+  - api_grid     — build-grid, save-grid, get-grid endpoints
+  - api_gutter   — gutter-repair endpoints
+  - api_box      — build-box-grids endpoints
+  - api_unified  — build-unified-grid endpoints
+  - helpers      — barrel re-export of all helper symbols
+  - columns      — column detection, cross-column splitting
+  - filters      — word/zone filtering, border ghosts
+  - headers      — header/heading detection, colspan detection
+  - zones        — vertical dividers, zone splitting/merging
+"""