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/middleware/errorHandler.js

@@ -26,23 +26,21 @@ const errorHandler = (err, req, res, next) => {
     timestamp: new Date().toISOString(),
   });
 
-  // Mongoose bad ObjectId
-  if (err.name === "CastError") {
+  // CouchDB document not found
+  if (err.name === "NotFoundError" || err.statusCode === 404) {
     const message = "Resource not found";
     error = new AppError(message, 404);
   }
 
-  // Mongoose duplicate key
-  if (err.code === 11000) {
+  // CouchDB conflict error (duplicate)
+  if (err.name === "ConflictError" || err.statusCode === 409) {
     const message = "Duplicate field value entered";
     error = new AppError(message, 400);
   }
 
-  // Mongoose validation error
-  if (err.name === "ValidationError") {
-    const message = Object.values(err.errors)
-      .map((val) => val.message)
-      .join(", ");
+  // CouchDB validation error
+  if (err.name === "ValidationError" || err.statusCode === 400) {
+    const message = err.message || "Validation failed";
     error = new AppError(message, 400);
   }