chore(lint): apply ruff --fix (4443 auto-fixes)

Auto-fixed categories:
- F401: unused imports removed
- I001: import blocks sorted
- W293: trailing whitespace stripped
- UP035: deprecated typing imports replaced
- SIM: simplify suggestions applied
- ARG: unused args prefixed with underscore
- T201: print statements removed
- F841: unused variables removed
- RUF059: unpacked variables prefixed

Remaining ~1500 unfixable errors (C901, B904, N806, E402) require manual work.

Backend smoke tests: 13/13 passed.

backend/src/api/routes/git/_merge_routes.py

@@ -2,25 +2,23 @@
 # @BRIEF FastAPI endpoints for merge operations (status, conflicts, resolve, abort, continue).
 # @LAYER: API
 
-from typing import List, Optional
 
 from fastapi import Depends, HTTPException
 
-from src.core.logger import belief_scope
-from src.dependencies import get_config_manager, has_permission
-
 from src.api.routes.git_schemas import (
     MergeConflictFileSchema,
     MergeContinueRequest,
     MergeResolveRequest,
     MergeStatusSchema,
 )
+from src.core.logger import belief_scope
+from src.dependencies import get_config_manager, has_permission
 
-from ._router import router
+from ._deps import get_git_service
 from ._helpers import (
     _handle_unexpected_git_route_error,
 )
-from ._deps import get_git_service
+from ._router import router
 
 
 # #region get_merge_status [C:2] [TYPE Function]
@@ -30,7 +28,7 @@ from ._deps import get_git_service
 )
 async def get_merge_status(
     dashboard_ref: str,
-    env_id: Optional[str] = None,
+    env_id: str | None = None,
     config_manager=Depends(get_config_manager),
     _=Depends(has_permission("plugin:git", "EXECUTE")),
 ):
@@ -54,11 +52,11 @@ async def get_merge_status(
 # @BRIEF Return conflicted files with mine/theirs previews for web conflict resolver.
 @router.get(
     "/repositories/{dashboard_ref}/merge/conflicts",
-    response_model=List[MergeConflictFileSchema],
+    response_model=list[MergeConflictFileSchema],
 )
 async def get_merge_conflicts(
     dashboard_ref: str,
-    env_id: Optional[str] = None,
+    env_id: str | None = None,
     config_manager=Depends(get_config_manager),
     _=Depends(has_permission("plugin:git", "EXECUTE")),
 ):
@@ -85,7 +83,7 @@ async def get_merge_conflicts(
 async def resolve_merge_conflicts(
     dashboard_ref: str,
     resolve_data: MergeResolveRequest,
-    env_id: Optional[str] = None,
+    env_id: str | None = None,
     config_manager=Depends(get_config_manager),
     _=Depends(has_permission("plugin:git", "EXECUTE")),
 ):
@@ -114,7 +112,7 @@ async def resolve_merge_conflicts(
 @router.post("/repositories/{dashboard_ref}/merge/abort")
 async def abort_merge(
     dashboard_ref: str,
-    env_id: Optional[str] = None,
+    env_id: str | None = None,
     config_manager=Depends(get_config_manager),
     _=Depends(has_permission("plugin:git", "EXECUTE")),
 ):
@@ -141,7 +139,7 @@ async def abort_merge(
 async def continue_merge(
     dashboard_ref: str,
     continue_data: MergeContinueRequest,
-    env_id: Optional[str] = None,
+    env_id: str | None = None,
     config_manager=Depends(get_config_manager),
     _=Depends(has_permission("plugin:git", "EXECUTE")),
 ):