fix: production deployment issues with Traefik and database migrations

- Add missing created_at column to email_queue table
- Fix 502 Bad Gateway errors with proper Traefik routing configuration
- Create docker-compose.traefik.yml for external Traefik deployment
- Fix health check endpoint path for API path stripping
- Add PostgreSQL init script for Umami database creation
- Add comprehensive deployment guide for Traefik setup

The backend now properly handles /api prefix stripping by Traefik and
migrations run safely in production environments with existing schemas.

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

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

backend/migrations/018_add_created_at_to_email_queue.js

@@ -0,0 +1,23 @@
+exports.up = async function(knex) {
+  // Check if created_at column already exists
+  const hasCreatedAt = await knex.schema.hasColumn('email_queue', 'created_at');
+  
+  if (!hasCreatedAt) {
+    await knex.schema.table('email_queue', (table) => {
+      table.datetime('created_at').defaultTo(knex.fn.now());
+    });
+    
+    // Update existing rows to have a created_at value based on scheduled_at
+    await knex('email_queue')
+      .whereNull('created_at')
+      .update({
+        created_at: knex.ref('scheduled_at')
+      });
+  }
+};
+
+exports.down = async function(knex) {
+  await knex.schema.table('email_queue', (table) => {
+    table.dropColumn('created_at');
+  });
+};