Fix code formatting and linting issues

- Auto-format all Python files with black and isort
- Remove unused imports with autoflake
- Fix flake8 issues (missing newlines, blank lines, etc.)
- Fix nonlocal/global scope issues in main.py
- Fix security.py import order (E402)
- Remove f-string without placeholders
- Add nosec comment for intentional exception handling
- Fix test imports to match refactored DMARCParser API

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/app/core/database.py

@@ -1,11 +1,10 @@
 from typing import Generator
 
+from app.core.config import get_settings
 from sqlalchemy import create_engine
 from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy.orm import sessionmaker
 
-from app.core.config import get_settings
-
 settings = get_settings()
 
 # Configure SQLAlchemy
@@ -24,4 +23,4 @@ def get_db() -> Generator:
     try:
         yield db
     finally:
-        db.close()
+        db.close()