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/core/middleware/trace.py

@@ -16,6 +16,7 @@ from starlette.types import ASGIApp
 
 from ..cot_logger import seed_trace_id, set_trace_id
 
+
 # #region TraceContextMiddleware [C:2] [TYPE Class] [SEMANTICS middleware, trace, context, dispatch]
 # @BRIEF Starlette BaseHTTPMiddleware that seeds a trace_id per request.
 class TraceContextMiddleware(BaseHTTPMiddleware):