Add flake8 config and fix linting issues

- Add .flake8 config with max-line-length=120 (compatible with Black)
- Remove unused imports (Dict, Any) from auth.py
- All tests pass locally: 21/21 unit tests, 69% coverage
- Black formatting check passes
- Flake8 linting passes with no errors

Author: mretallack

.flake8

@@ -0,0 +1,14 @@
+[flake8]
+max-line-length = 120
+extend-ignore = E203, W503
+exclude =
+    .git,
+    __pycache__,
+    build,
+    dist,
+    .eggs,
+    *.egg-info,
+    .venv,
+    venv,
+    .pytest_cache,
+    htmlcov

ukfuelfinder/auth.py

@@ -4,7 +4,7 @@
 
 import time
 import threading
-from typing import Optional, Dict, Any
+from typing import Optional
 import requests
 from .exceptions import AuthenticationError, InvalidCredentialsError