Enforce uppercase README.md and TODO.md filenames

- Renamed all Readme.md → README.md
- Renamed all Todo.md → TODO.md
- Added integration test (tests/integration/test_filename_conventions.py) to automatically check naming convention.

Background:
Consistency in file naming (uppercase README.md and TODO.md) avoids issues with case-sensitive filesystems and ensures desktop cards (e.g. Pretix) are properly included.
Ref: https://chatgpt.com/share/68b1d135-c688-800f-9441-46a3cbfee175

tests/integration/test_filename_conventions.py

@@ -0,0 +1,27 @@
+import os
+import unittest
+
+class TestFilenameConventions(unittest.TestCase):
+    """
+    Integration test to ensure README.md and TODO.md files
+    are always written in uppercase (README.md / TODO.md).
+    """
+
+    def test_readme_and_todo_filenames_are_uppercase(self):
+        bad_files = []
+        for root, _, files in os.walk("."):
+            for filename in files:
+                lower = filename.lower()
+                if lower in ("readme.md", "todo.md"):
+                    if filename not in ("README.md", "TODO.md"):
+                        bad_files.append(os.path.join(root, filename))
+
+        msg = (
+            "The following files violate uppercase naming convention "
+            "(must be README.md or TODO.md):\n- " + "\n- ".join(bad_files)
+        ) if bad_files else None
+
+        self.assertEqual(bad_files, [], msg)
+
+if __name__ == "__main__":
+    unittest.main()