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/build/__init__.py

@@ -0,0 +1,11 @@
+"""
+Grid Build sub-package — grid construction pipeline.
+
+Modules:
+  - core      — _build_grid_core() main entry point
+  - zones     — image loading, graphic/box detection, zone-aware grid building
+  - cleanup   — junk row removal, artifact cleanup, pipe dividers
+  - text_ops  — color annotation, heading detection, IPA correction
+  - cell_ops  — bullet removal, garbled cells, word-box reordering
+  - finalize  — dictionary detection, spell checking, result assembly
+"""