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

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

backend/services/couchdbService.js

@@ -430,18 +430,20 @@ class CouchDBService {
         // Check if design document exists
         const existing = await this.getDocument(designDoc._id);
         
-        // Update with new revision
-        designDoc._rev = existing._rev;
-        await this.makeRequest('PUT', `/${this.dbName}/${designDoc._id}`, designDoc);
-        console.log(`Updated design document: ${designDoc._id}`);
-      } catch (error) {
-        if (error.statusCode === 404) {
-          // Create new design document
+        if (existing) {
+          // Update with new revision
+          designDoc._rev = existing._rev;
           await this.makeRequest('PUT', `/${this.dbName}/${designDoc._id}`, designDoc);
-          console.log(`Created design document: ${designDoc._id}`);
+          console.log(`Updated design document: ${designDoc._id}`);
         } else {
-          console.error(`Error creating design document ${designDoc._id}:`, error.message);
+          // Create new design document
+          const designDocToCreate = { ...designDoc };
+          delete designDocToCreate._rev;
+          await this.makeRequest('PUT', `/${this.dbName}/${designDoc._id}`, designDocToCreate);
+          console.log(`Created design document: ${designDoc._id}`);
         }
+      } catch (error) {
+        console.error(`Error creating design document ${designDoc._id}:`, error.message);
       }
     }
   }