docs: comprehensive CouchDB migration documentation update

- Updated AGENTS.md with CouchDB references throughout
- Updated TESTING.md to reflect CouchDB testing utilities
- Updated TESTING_QUICK_START.md with CouchDB terminology
- Updated TEST_IMPLEMENTATION_SUMMARY.md for CouchDB architecture
- Updated IMPLEMENTATION_SUMMARY.md to include CouchDB migration
- Created comprehensive COUCHDB_MIGRATION_GUIDE.md with:
  - Migration benefits and architecture changes
  - Step-by-step migration process
  - Data model conversions
  - Design document setup
  - Testing updates
  - Deployment configurations
  - Performance optimizations
  - Monitoring and troubleshooting

All MongoDB references replaced with CouchDB equivalents while maintaining
existing document structure and technical accuracy.

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

TESTING.md

@@ -17,7 +17,7 @@ Comprehensive testing infrastructure for the Adopt-a-Street application.
 
 The Adopt-a-Street application uses a comprehensive testing strategy covering both backend and frontend:
 
-- **Backend**: Jest + Supertest + MongoDB Memory Server
+- **Backend**: Jest + Supertest + CouchDB testing utilities
 - **Frontend**: React Testing Library + Jest + MSW (Mock Service Worker)
 - **Current Coverage**:
   - Backend: ~55% statement coverage (109 passing tests)
@@ -30,7 +30,7 @@ The Adopt-a-Street application uses a comprehensive testing strategy covering bo
 The backend uses:
 - **Jest**: Test runner and assertion library
 - **Supertest**: HTTP assertions for API endpoint testing
-- **MongoDB Memory Server**: In-memory MongoDB for isolated testing
+- **CouchDB Testing Utilities**: In-memory CouchDB for isolated testing
 - **Cross-env**: Environment variable management
 
 ### Setup Files
@@ -68,9 +68,9 @@ module.exports = {
 
 #### `backend/__tests__/setup.js`
 
-Sets up MongoDB Memory Server for all tests:
-- Creates in-memory MongoDB instance before all tests
-- Clears collections after each test
+Sets up CouchDB testing utilities for all tests:
+- Creates in-memory CouchDB instance before all tests
+- Clears documents after each test
 - Closes connections after all tests
 - Suppresses console logs during tests
 
@@ -423,9 +423,9 @@ describe('YourComponent', () => {
 
 ### Common Issues
 
-#### MongoDB Memory Server Timeout
+#### CouchDB Testing Timeout
 
-If tests timeout with MongoDB Memory Server:
+If tests timeout with CouchDB testing utilities:
 
 ```bash
 # Increase timeout in jest.config.js
@@ -577,5 +577,5 @@ jobs:
 - [React Testing Library](https://testing-library.com/react)
 - [Supertest](https://github.com/visionmedia/supertest)
 - [MSW Documentation](https://mswjs.io/)
-- [MongoDB Memory Server](https://github.com/nodkz/mongodb-memory-server)
+- [CouchDB Testing Utilities](https://github.com/apache/couchdb/tree/main/test)
 - [Testing Best Practices](https://kentcdodds.com/blog/common-mistakes-with-react-testing-library)