feat: pre-zip download all and photo replacement by name (#312, #313)

Pre-zip downloads:
- Generate ZIP in background after photo mutations (upload/delete/watermark change)
- Serve cached zip with Content-Length for instant downloads and native progress bar
- Falls back to on-the-fly streaming when no cache exists yet
- Frontend uses browser-native download when zip is ready (no blob buffering)
- New downloadZipService with debounced regeneration and in-memory locking

Photo replacement:
- Admin upload form gets "Replace existing photos with same name" checkbox
- Matches by original_filename (case-insensitive) within the same event
- Preserves photo ID, position, feedback, category, and visibility
- Updates file, thumbnail, dimensions, EXIF capture date on replacement
- Ambiguous matches (multiple photos with same name) skip replacement with warning
- New photoReplacementService with findReplacementCandidate and replacePhoto

backend/migrations/core/079_add_download_zip_cache.js

@@ -0,0 +1,26 @@
+/**
+ * Add download ZIP cache columns to events table.
+ *
+ * Enables pre-generated ZIP files for "Download All" so guests get
+ * instant downloads with Content-Length instead of on-the-fly streaming.
+ */
+
+exports.up = async function(knex) {
+  const hasZipPath = await knex.schema.hasColumn('events', 'download_zip_path');
+  if (!hasZipPath) {
+    await knex.schema.alterTable('events', (table) => {
+      table.text('download_zip_path').nullable().defaultTo(null);
+      table.datetime('download_zip_generated_at').nullable().defaultTo(null);
+    });
+  }
+};
+
+exports.down = async function(knex) {
+  const hasZipPath = await knex.schema.hasColumn('events', 'download_zip_path');
+  if (hasZipPath) {
+    await knex.schema.alterTable('events', (table) => {
+      table.dropColumn('download_zip_path');
+      table.dropColumn('download_zip_generated_at');
+    });
+  }
+};