feat: implement dynamic rate limiting with database configuration

- Add database migration for rate limit settings
- Create rate limit service with dynamic configuration from database
- Implement proper authentication detection for admin and gallery tokens
- Skip rate limiting for authenticated users (configurable)
- Add admin API endpoint to update rate limit settings
- Use correct client IP detection with proxy support
- Cache settings for performance (1 minute cache)
- Default to 1000 requests per 15 minutes for better UX
- Apply auth-specific limits only to login endpoints

Key improvements:
- No more rate limiting for authenticated gallery/admin users
- Configurable via admin settings page
- Immediate effect when settings change
- Better handling of proxied requests

🤖 Generated with [Claude Code](https://claude.ai/code)

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

backend/migrations/027_add_rate_limit_settings.js

@@ -0,0 +1,63 @@
+exports.up = async function(knex) {
+  // Add rate limit settings to app_settings
+  const rateLimitSettings = [
+    {
+      setting_key: 'rate_limit_enabled',
+      setting_value: JSON.stringify(true),
+      setting_type: 'security'
+    },
+    {
+      setting_key: 'rate_limit_window_minutes',
+      setting_value: JSON.stringify(15),
+      setting_type: 'security'
+    },
+    {
+      setting_key: 'rate_limit_max_requests',
+      setting_value: JSON.stringify(1000),
+      setting_type: 'security'
+    },
+    {
+      setting_key: 'rate_limit_auth_max_requests',
+      setting_value: JSON.stringify(5),
+      setting_type: 'security'
+    },
+    {
+      setting_key: 'rate_limit_skip_authenticated',
+      setting_value: JSON.stringify(true),
+      setting_type: 'security'
+    },
+    {
+      setting_key: 'rate_limit_public_endpoints_only',
+      setting_value: JSON.stringify(false),
+      setting_type: 'security'
+    }
+  ];
+
+  // Insert settings if they don't exist
+  for (const setting of rateLimitSettings) {
+    const exists = await knex('app_settings')
+      .where('setting_key', setting.setting_key)
+      .first();
+    
+    if (!exists) {
+      await knex('app_settings').insert({
+        ...setting,
+        updated_at: knex.fn.now()
+      });
+    }
+  }
+};
+
+exports.down = async function(knex) {
+  // Remove rate limit settings
+  await knex('app_settings')
+    .whereIn('setting_key', [
+      'rate_limit_enabled',
+      'rate_limit_window_minutes',
+      'rate_limit_max_requests',
+      'rate_limit_auth_max_requests',
+      'rate_limit_skip_authenticated',
+      'rate_limit_public_endpoints_only'
+    ])
+    .del();
+};