feat: add strict mypy type-checking for app/utils/ module

- Add [[tool.mypy.overrides]] section for app/utils/** with disallow_untyped_defs=true
- Add type annotations to all functions in app/utils/ (12 files)
- Fix type annotations in app/config.py and app/database.py (imported by utils)
- All 85 source files now pass mypy type checking

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

app/utils/encryption.py

@@ -16,7 +16,7 @@ logger = logging.getLogger(__name__)
 _cipher_suite = None
 
 
-def _get_cipher_suite():
+def _get_cipher_suite() -> object | None:
     """
     Get or create the Fernet cipher suite for encryption/decryption.