Added mypy type checking as a cicd pipe I noticed some type errors.

There are some errors but should be relatively easy to fix!

Author: semohr

.github/workflows/python.yml

@@ -24,12 +24,16 @@ jobs:
                   cd ./backend
                   python -m pip install --upgrade pip
                   pip install ruff
-
-            # Update output format to enable automatic inline annotations.
+                  pip install mypy
             - name: Check style with Ruff
               run: |
                   cd ./backend
                   ruff check --output-format=github .
+            - name: Check type hints with mypy
+              run: |
+                  cd ./backend
+                  pip install .
+                  mypy --show-error-codes beets_flask
             - name: Test with pytest
               run: |
                   cd ./backend

backend/pyproject.toml

@@ -83,3 +83,8 @@ pythonpath = ["."]
 
 [tool.coverage.report]
 omit = ["*/tests/*"]
+
+
+[tool.mypy]
+ignore_missing_imports = true
+check_untyped_defs = true