refactor: remove MongoDB legacy code and references

- Archive migration script to scripts/archive/migrate-to-couchdb.js
- Update error handler middleware for CouchDB-appropriate errors
- Fix MongoDB references in test utilities and comments
- Replace MongoDB ObjectId references with CouchDB ID patterns
- Preserve existing functionality while removing legacy dependencies

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backend/__tests__/utils/idGenerator.js

@@ -1,11 +1,11 @@
 /**
  * Utility functions for generating test IDs
- * Replaces mongoose.Types.ObjectId() functionality
+ * Replaces legacy ID generation functionality
  */
 
 /**
  * Generate a random test ID string
- * Format: random alphanumeric string (24 characters like MongoDB ObjectId)
+ * Format: random alphanumeric string (24 characters)
  */
 function generateTestId() {
   const chars = 'abcdefghijklmnopqrstuvwxyz0123456789';