refactor(backend): add status constants and update code to use them

- Create constants/statuses.js with MATCH_STATUS, SUGGESTION_STATUS
- Update routes/dashboard.js to use MATCH_STATUS
- Update routes/matches.js to use MATCH_STATUS
- Update routes/events.js to use SUGGESTION_STATUS
- Update services/matching.js to use SUGGESTION_STATUS
- Update tests to use constants

backend/src/constants/index.js

@@ -0,0 +1,6 @@
+const { MATCH_STATUS, SUGGESTION_STATUS } = require('./statuses');
+
+module.exports = {
+  MATCH_STATUS,
+  SUGGESTION_STATUS,
+};