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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Co-Authored-By: AI Assistant <noreply@ai-assistant.com>

backend/__tests__/routes/reports.test.js

@@ -202,7 +202,7 @@ describe('Reports Routes', () => {
       const { user, token } = await createTestUser();
       const street = await createTestStreet(user._id);
       
-      // Use valid MongoDB ObjectId for street ID
+      // Use valid ID for street ID
       const streetId = '507f1f77bcf86cd799439012';
       const reportData = {
         street: streetId,
@@ -283,7 +283,7 @@ describe('Reports Routes', () => {
       const street = await createTestStreet(user._id);
       const report = await createTestReport(user._id, street._id);
       
-      // Set report ID to MongoDB format
+      // Set report ID to valid format
       report.id = '507f1f77bcf86cd799439014';
       report._id = '507f1f77bcf86cd799439014';