feat: Complete critical CouchDB migration fixes and infrastructure improvements

- Fix design document initialization with proper null handling
- Fix bulk operations in migration script (bulkDocs method signature)
- Remove hardcoded credentials from docker-compose.yml
- Fix test infrastructure incompatibility (use npm/Jest instead of bun)
- Implement comprehensive database indexes for performance
- Add health check endpoint for Docker container monitoring
- Create 7 design documents: users, streets, tasks, posts, badges, transactions, general
- Update jest.setup.js with proper mock exports
- Add .env.example with secure defaults

🤖 Generated with [AI Assistant]

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

backend/__tests__/jest.setup.js

@@ -1,5 +1,5 @@
 // Mock CouchDB service globally for all tests
-jest.mock('../services/couchdbService', () => ({
+const mockCouchdbService = {
   initialize: jest.fn().mockResolvedValue(true),
   isReady: jest.fn().mockReturnValue(true),
   isConnected: true,
@@ -27,9 +27,15 @@ jest.mock('../services/couchdbService', () => ({
   update: jest.fn(),
   updateUserPoints: jest.fn(),
   getDocument: jest.fn(),
+  findDocumentById: jest.fn(),
   bulkDocs: jest.fn(),
   shutdown: jest.fn().mockResolvedValue(true),
-}));
+};
+
+jest.mock('../services/couchdbService', () => mockCouchdbService);
+
+// Make the mock available for tests to reference
+global.mockCouchdbService = mockCouchdbService;
 
 // Set test environment variables
 process.env.JWT_SECRET = 'test-jwt-secret';