fix: correct import path and minor issues in quartoFlavor update

- Fix wrong import path '../models/Project.mjs' → '../../models/Project.mjs'
  (from Features/Compile/, '..' is Features/, not src/; the server would
  crash on startup with ERR_MODULE_NOT_FOUND in Node.js ESM)
- Log MongoDB errors instead of silently swallowing them
- Remove null from Mongoose String enum (not a valid enum value for strings)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/web/app/src/models/Project.mjs

@@ -38,7 +38,7 @@ export const ProjectSchema = new Schema(
     version: { type: Number }, // incremented for every change in the project structure (folders and filenames)
     publicAccesLevel: { type: String, default: 'private' },
     compiler: { type: String, default: settings.defaultLatexCompiler },
-    quartoFlavor: { type: String, enum: ['revealjs', 'pdf', null] },
+    quartoFlavor: { type: String, enum: ['revealjs', 'pdf'] },
     spellCheckLanguage: { type: String, default: 'en' },
     deletedByExternalDataSource: { type: Boolean, default: false },
     description: { type: String, default: '' },