feat: Add database import/export functionality (Issue #10)

Backend:
- New /api/v1/backup/export endpoint - exports all data as single JSON
- New /api/v1/backup/import endpoint - imports JSON with merge or replace
- Handles vendors, filaments, spools, adjustments, settings
- Remaps IDs to maintain relationships during import

Frontend:
- Added Backup & Restore section to Settings page
- Download Backup button for export
- Import with merge or replace options
- Warning confirmation for replace mode

This enables migrating between PostgreSQL and SQLite databases.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

client/public/locales/en/common.json

@@ -348,6 +348,22 @@
                 "tooltip": "Round prices to the nearest whole number."
             }
         },
+        "backup": {
+            "title": "Backup & Restore",
+            "description": "Export all your data for backup or migrate to a different database.",
+            "export_title": "Export Data",
+            "export_button": "Download Backup",
+            "export_success": "Backup downloaded successfully",
+            "export_error": "Failed to export data",
+            "import_title": "Import Data",
+            "select_file": "Select backup file",
+            "import_merge": "Import (Merge)",
+            "import_replace": "Import (Replace All)",
+            "import_success": "Data imported successfully",
+            "import_error": "Failed to import data",
+            "replace_warning_title": "Replace all data?",
+            "replace_warning": "This will DELETE all existing data and replace it with the backup. This cannot be undone!"
+        },
         "extra_fields": {
             "tab": "Extra Fields",
             "description": "<p>Here you can add extra custom fields to your entities.</p><p>Once a field is added, you can not change its key or type, and for choice type fields you can not remove choices or change the multi choice state. If you remove a field, the associated data for all entities will be deleted.</p><p>The key is what other programs read/write the data as, so if your custom field is supposed to integrate with a third-party program, make sure to set it correctly. Default value is only applied to new items.</p><p>Extra fields can not be sorted or filtered in the table views.</p>",