feat: add edit artist endpoint (section 1.4)

- Add GET/PUT /api/artists/{id} for artist settings
- Update sync to create artists table entry (library settings)
- Support partial updates for monitored, path, quality/metadata profiles
- Add e2e tests for get, edit, partial update flows

testing/e2e/testutil/testutil.go

@@ -188,6 +188,11 @@ func (e *TestEnv) DELETE(path string) (*APIResponse, error) {
 	return e.Do(APIRequest{Method: http.MethodDelete, Path: path})
 }
 
+// PUT is a convenience method for PUT requests.
+func (e *TestEnv) PUT(path string, body any) (*APIResponse, error) {
+	return e.Do(APIRequest{Method: http.MethodPut, Path: path, Body: body})
+}
+
 // DecodeJSON decodes the response body into the given value.
 func (r *APIResponse) DecodeJSON(v any) error {
 	return json.Unmarshal(r.Body, v)