Only allow supported search types to /search, /regenerate APIs

- Use a SearchType to limit types that can be passed by user
- FastAPI automatically validates type passed in query param
- Available type options show up in Swagger UI, FastAPI docs
- controller code looks neater instead of doing string comparisons for type
- Test invalid, valid search types via pytest

src/utils/config.py

@@ -0,0 +1,9 @@
+from enum import Enum
+
+
+class SearchType(str, Enum):
+    Notes = "notes"
+    Ledger = "ledger"
+    Music = "music"
+    Image = "image"
+