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/test_pat_api.py

@@ -1,5 +1,6 @@
+
 from pydantic import BaseModel, Field
-from typing import Optional
+
 
 class GitProvider(str):
     pass
@@ -10,8 +11,8 @@ class GitServerConfigBase(BaseModel):
     url: str = Field(..., description="Server base URL")
     pat: str = Field(..., description="Personal Access Token")
     pat: str = Field(..., description="Personal Access Token")
-    default_repository: Optional[str] = Field(None, description="Default repository path (org/repo)")
-    default_branch: Optional[str] = Field("main", description="Default branch logic/name")
+    default_repository: str | None = Field(None, description="Default repository path (org/repo)")
+    default_branch: str | None = Field("main", description="Default branch logic/name")
 
 class GitServerConfigSchema(GitServerConfigBase):
     id: str