[split-required] Split remaining Python monoliths (Phase 1 continued)

klausur-service (7 monoliths):
- grid_editor_helpers.py (1,737 → 5 files: columns, filters, headers, zones)
- cv_cell_grid.py (1,675 → 7 files: build, legacy, streaming, merge, vocab)
- worksheet_editor_api.py (1,305 → 4 files: models, AI, reconstruct, routes)
- legal_corpus_ingestion.py (1,280 → 3 files: registry, chunking, ingestion)
- cv_review.py (1,248 → 4 files: pipeline, spell, LLM, barrel)
- cv_preprocessing.py (1,166 → 3 files: deskew, dewarp, barrel)
- rbac.py, admin_api.py, routes/eh.py remain (next batch)

backend-lehrer (1 monolith):
- classroom_engine/repository.py (1,705 → 7 files by domain)

All re-export barrels preserve backward compatibility.
Zero import errors verified.

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

.claude/rules/loc-exceptions.txt

@@ -17,6 +17,7 @@
 
 # Pure Data Registries (keine Logik, nur Daten-Definitionen)
 **/dsfa_sources_registry.py | owner=klausur | reason=Pure data registry (license + source definitions, no logic) | review=2027-01-01
+**/legal_corpus_registry.py | owner=klausur | reason=Pure data registry (Regulation dataclass + 47 regulation definitions, no logic) | review=2027-01-01
 **/backlog/backlog-items.ts | owner=admin-lehrer | reason=Pure data array (506 LOC, no logic, only BacklogItem[] literals) | review=2027-01-01
 **/lib/module-registry-data.ts | owner=admin-lehrer | reason=Pure data array (510 LOC, no logic, only BackendModule[] literals) | review=2027-01-01