refactor(logging): replace structlog with stdlib logging compat layer

- Remove structlog dependency from backend/pyproject.toml
- Add app.utils.logging_compat shim for keyword-arg logging API
- Add app.utils.json_formatter for JSON log output with extra fields
- Update all backend modules to use logging_compat.get_logger()
- Update docstrings in log_sanitizer.py and json_formatter.py
- Update test comment in test_async_utils.py
- Record 406 failing tests in Docs/Tasks.md for tracking

backend/app/middleware/metrics.py

@@ -10,7 +10,7 @@ import re
 import time
 from typing import TYPE_CHECKING
 
-import structlog
+from app.utils.logging_compat import get_logger
 from starlette.middleware.base import BaseHTTPMiddleware
 
 from app.utils.metrics import http_active_requests, http_request_count, http_request_latency
@@ -21,7 +21,7 @@ if TYPE_CHECKING:
     from starlette.requests import Request
     from starlette.responses import Response
 
-log = structlog.get_logger()
+log = get_logger(__name__)
 
 # Paths excluded from detailed metrics (to avoid cardinality explosion)
 EXCLUDED_PATHS = {"/metrics", "/health", "/api/health"}