refactor: decompose 6 monolithic modules (>1000 LOC) into domain packages

Decompose 6 large modules into packages with <400-line modules to satisfy INV_7:

- superset_client.py (2145->0) -> superset_client/ package (12 modules)
- assistant.py (2683->0) -> assistant/ package (12 modules)
- git_service.py (2101->11 shim) -> services/git/ package (9 modules)
- git.py (1757->0) -> routes/git/ package (11 modules)
- dashboards.py (1619->0) -> routes/dashboards/ package (8 modules)
- translate.py (1587->0) -> routes/translate/ package (11 modules)
- superset_context_extractor.py (1397->0) -> utils/superset_context_extractor/ (7 modules)

All modules <400 lines (INV_7). All 80 tests pass.
All external imports preserved via __init__.py re-exports or shim.
Semantic [DEF] contracts with @LAYER/@SEMANTICS added to all Module types.

backend/src/api/routes/assistant/_dispatch.py

@@ -36,7 +36,7 @@ from ._resolvers import (
 )
 from ._history import _coerce_query_bool
 from ._llm_planner import _check_any_permission
-from ._dataset_review import _dispatch_dataset_review_intent
+from ._dataset_review_dispatch import _dispatch_dataset_review_intent
 
 git_service = GitService()