feat: add original filename preservation and Lightroom export support

Addresses GitHub issue #132 - enables filtering client feedback and
exporting filenames for use in Lightroom.

Changes:
- Add original_filename column to photos table via migration
- Store original filename during photo upload
- Fix export service column name mismatches (path, size_bytes, uploaded_at)
- Fix table name (photo_categories instead of categories)
- Fix toFixed() calls to handle string ratings from database

Export formats available:
- TXT with comma separator (for Lightroom Library Filter)
- CSV with full metadata
- JSON for automation
- XMP sidecar files (for Lightroom/Bridge/Capture One)

backend/migrations/core/062_add_original_filename.js

@@ -0,0 +1,23 @@
+/**
+ * Migration 062: Add original_filename to photos table
+ * - photos.original_filename: preserves the original filename from upload
+ *   This enables Lightroom integration by exporting filtered filenames
+ */
+
+const { addColumnIfNotExists } = require('../helpers');
+
+exports.up = async function(knex) {
+  console.log('Running migration: 062_add_original_filename');
+
+  // photos.original_filename (nullable - original filename before renaming)
+  await addColumnIfNotExists(knex, 'photos', 'original_filename', (table) => {
+    table.string('original_filename', 512);
+  });
+
+  console.log('Migration 062_add_original_filename completed');
+};
+
+exports.down = async function(knex) {
+  console.log('Rollback: 062_add_original_filename');
+  // Keep column (safe rollback not removing data). Intentionally no-op.
+};