feat: implement comprehensive CouchDB service and migration utilities

- Add production-ready CouchDB service with connection management
- Implement design documents with views and Mango indexes
- Create CRUD operations with proper error handling
- Add specialized helper methods for all document types
- Include batch operations and conflict resolution
- Create comprehensive migration script from MongoDB to CouchDB
- Add test suite with graceful handling when CouchDB unavailable
- Include detailed documentation and usage guide
- Update environment configuration for CouchDB support
- Follow existing code patterns and conventions

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

backend/package.json

@@ -27,8 +27,8 @@
     "globals": "^16.4.0",
     "helmet": "^8.1.0",
     "jsonwebtoken": "^9.0.2",
-    "mongoose": "^8.12.1",
     "multer": "^1.4.5-lts.1",
+    "nano": "^10.1.4",
     "socket.io": "^4.8.1",
     "stripe": "^17.7.0"
   },
@@ -37,7 +37,6 @@
     "cross-env": "^10.1.0",
     "eslint": "^9.38.0",
     "jest": "^30.2.0",
-    "mongodb-memory-server": "^10.3.0",
     "supertest": "^7.1.4"
   }
 }