feat: add configurable upload batch size for reverse proxy compatibility (#208)

Users behind Cloudflare Tunnel and other reverse proxies cannot upload
batches >100MB. The upload chunking previously used a hardcoded 500MB
limit. This adds a configurable `max_upload_batch_size_mb` setting
(default 95MB) to the admin General settings, leaving headroom below
Cloudflare's 100MB limit.

backend/migrations/core/072_add_max_upload_batch_size.js

@@ -0,0 +1,20 @@
+exports.up = async function(knex) {
+  const exists = await knex('app_settings')
+    .where({ setting_key: 'general_max_upload_batch_size_mb' })
+    .first();
+
+  if (!exists) {
+    await knex('app_settings').insert({
+      setting_key: 'general_max_upload_batch_size_mb',
+      setting_value: JSON.stringify(95),
+      setting_type: 'general',
+      updated_at: new Date()
+    });
+  }
+};
+
+exports.down = async function(knex) {
+  await knex('app_settings')
+    .where({ setting_key: 'general_max_upload_batch_size_mb' })
+    .del();
+};