Auto-run database migrations in generated spec_helper

Generated apps no longer need a manual AMBER_ENV=test migration step
before running crystal spec — the spec helper now points Micrate at the
configured database and runs pending migrations automatically.

Carried forward from amberframework/amber#1382 by @renich (Renich Bon
Ciric), who identified and fixed this in the v1 bundled CLI; the bundled
CLI's templates moved here in V2, so the fix lands where new apps are
actually generated.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: crimson-knight

src/amber_cli/templates/app/spec/spec_helper.cr.ecr

@@ -5,6 +5,6 @@ require "micrate"
 
 require "../config/application"
 
-# Micrate::DB.connection_url = ENV["DATABASE_URL"]? || Amber.settings.database_url
+Micrate::DB.connection_url = ENV["DATABASE_URL"]? || Amber.settings.database_url
 # Automatically run migrations on the test database
-# Micrate::Cli.run_up
+Micrate::Cli.run_up