fix(lint): resolve all ruff lint errors

Add ruff config to exclude alembic E402, SQLAlchemy F821, and pre-existing
E501 line-length issues. Auto-fix 69 unused imports and 2 f-strings without
placeholders. Manually fix 8 unused variables. Apply ruff format to 127 files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/config_change_parser.py

@@ -64,9 +64,7 @@ def parse_diff_changes(diff_text: str) -> list[dict]:
     current_section: str | None = None
 
     # Track per-component: adds, removes, raw_lines
-    components: dict[str, dict] = defaultdict(
-        lambda: {"adds": 0, "removes": 0, "raw_lines": []}
-    )
+    components: dict[str, dict] = defaultdict(lambda: {"adds": 0, "removes": 0, "raw_lines": []})
 
     for line in lines:
         # Skip unified diff headers