Enhance email templates with clickable links, branding, and improved styling

- Add clickable gallery links in all email templates
- Include application logo in email header and footer (custom or PicPeak default)
- Redesign emails with professional styling matching gallery login page
  - Gray background with white content box
  - PicPeak green header with centered logo
  - Clean typography and proper spacing
  - Responsive design for mobile devices
  - Styled call-to-action buttons
  - Footer with branding and copyright
- Update email processor to fetch branding settings dynamically
- Use proper API URLs for logo images in emails

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

Co-Authored-By: Claude <[email protected]>

backend/migrations/012_add_hero_photo_id.js

@@ -0,0 +1,15 @@
+exports.up = async function(knex) {
+  // Add hero_photo_id to events table
+  const hasColumn = await knex.schema.hasColumn('events', 'hero_photo_id');
+  if (!hasColumn) {
+    await knex.schema.alterTable('events', function(table) {
+      table.integer('hero_photo_id').references('id').inTable('photos').onDelete('SET NULL');
+    });
+  }
+};
+
+exports.down = async function(knex) {
+  await knex.schema.alterTable('events', function(table) {
+    table.dropColumn('hero_photo_id');
+  });
+};