feat: document versioning, templates, ZIP backup/restore, and Open/Edit workflow

Backend:
- schema.js: add document_templates, template_versions, document_versions tables;
  migrate documents with template_id/template_version_id columns;
  auto-seed Version 1 for all 190 existing documents
- templateController.js (NEW): full CRUD + file upload/download for templates
  and template versions
- templates.js route (NEW): /api/templates endpoints
- documentController.js: add getDocumentVersions, uploadDocumentVersion,
  downloadDocumentVersion; uploadDocument now seeds Version 1 on creation
- documents.js: wire GET /:id/versions, POST /:id/upload-version
- backupController.js: upgrade backup to ZIP archive (DB + student files +
  template files + SHA-256 manifest); restore handles ZIP and legacy .db;
  safety backup taken before every restore
- package.json: add archiver + unzipper dependencies

Frontend:
- template_model.dart (NEW): DocumentTemplateModel + TemplateVersionModel entities
- document_version_model.dart (NEW): DocumentVersionModel entity
- template_repository.dart (NEW): Dio-based template API client
- document_repository.dart: add getDocumentVersions, uploadDocumentVersion,
  downloadDocumentVersion methods
- document_preview_modal.dart: add Open / Open With (Windows rundll32
  OpenAs_RunDLL / Android chooser) / Edit (download-temp -> open -> upload
  prompt) / Upload New Version (FilePicker) / Version History actions
- document_version_history_sheet.dart (NEW): draggable bottom sheet listing
  all document versions with per-version Preview + Download
- file_folder_card.dart: add Open With, Edit, Upload New Version, Version
  History items to context menu
- documents_screen.dart: wire open_with, edit, upload_version, version_history,
  restore action cases; import version history sheet

Bugfix:
- student_profile_modal.dart: remove enrollment.status access that caused
  NoSuchMethodError (enrollments table has no status column)

backend/package.json

@@ -19,6 +19,7 @@
   "license": "ISC",
   "type": "commonjs",
   "dependencies": {
+    "archiver": "^8.0.0",
     "bcrypt": "^6.0.0",
     "better-sqlite3": "^12.9.0",
     "cors": "^2.8.6",
@@ -30,7 +31,8 @@
     "morgan": "^1.10.1",
     "multer": "^2.1.1",
     "nodemailer": "^9.0.6",
-    "sharp": "^0.35.4"
+    "sharp": "^0.35.4",
+    "unzipper": "^0.12.5"
   },
   "devDependencies": {
     "nodemon": "^3.1.14"