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/git/__init__.py

@@ -0,0 +1,44 @@
+# [DEF:GitPackage:Module]
+# @COMPLEXITY: 3
+# @PURPOSE: Package root for decomposed git routes. Re-exports all public symbols from submodules.
+# @LAYER: API
+# @SEMANTICS: git, package, re-exports
+# @RELATION: USES -> [GitRouter, GitDeps, GitHelpers, GitConfigRoutes, GitGiteaRoutes,
+#                     GitRepoRoutes, GitRepoOperationsRoutes, GitRepoLifecycleRoutes,
+#                     GitMergeRoutes, GitEnvironmentRoutes]
+# @INVARIANT: git_service and os are module-level attributes for test monkeypatch compatibility.
+#             All route functions are re-exported for direct access via `from src.api.routes import git`.
+
+import os
+
+from src.services.git_service import GitService
+
+from ._router import router
+from ._deps import MAX_REPOSITORY_STATUS_BATCH
+
+# -- git_service singleton (canonical instance; tests may monkeypatch git_routes.git_service) --
+git_service = GitService()
+
+# -- Config routes --
+from ._config_routes import get_git_configs, create_git_config, update_git_config, delete_git_config, test_git_config  # noqa: E501, F401
+
+# -- Gitea routes --
+from ._gitea_routes import list_gitea_repositories, create_gitea_repository, delete_gitea_repository, create_remote_repository  # noqa: E501, F401
+
+# -- Repo routes (core) --
+from ._repo_routes import init_repository, get_repository_binding, delete_repository, get_branches, create_branch, checkout_branch  # noqa: E501, F401
+from ._helpers import _resolve_dashboard_id_from_ref, _resolve_dashboard_id_from_ref_async, _resolve_repo_key_from_ref  # noqa: E501, F401 — re-exported for test monkeypatch compatibility
+
+# -- Repo operations routes (commit, push, pull, status, diff, history, generate-message) --
+from ._repo_operations_routes import commit_changes, push_changes, pull_changes, get_repository_status, get_repository_status_batch, get_repository_diff, get_history, generate_commit_message  # noqa: E501, F401
+
+# -- Repo lifecycle routes (sync, promote, deploy) --
+from ._repo_lifecycle_routes import sync_dashboard, promote_dashboard, deploy_dashboard  # noqa: E501, F401
+
+# -- Merge routes --
+from ._merge_routes import get_merge_status, get_merge_conflicts, resolve_merge_conflicts, abort_merge, continue_merge  # noqa: E501, F401
+
+# -- Environment routes --
+from ._environment_routes import get_environments  # noqa: E501, F401
+
+# [/DEF:GitPackage:Module]