fix: PostgreSQL trigger syntax (remove IF NOT EXISTS)

PostgreSQL does not support IF NOT EXISTS for CREATE TRIGGER.
Use DROP TRIGGER IF EXISTS before CREATE TRIGGER instead.

Fixes: Backend crash loop due to schema.sql syntax error on line 231

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/schema.sql

@@ -228,12 +228,14 @@ BEGIN
 END;
 $$ LANGUAGE plpgsql;
 
-CREATE TRIGGER IF NOT EXISTS trigger_profiles_updated
+DROP TRIGGER IF EXISTS trigger_profiles_updated ON profiles;
+CREATE TRIGGER trigger_profiles_updated
     BEFORE UPDATE ON profiles
     FOR EACH ROW
     EXECUTE FUNCTION update_updated_timestamp();
 
-CREATE TRIGGER IF NOT EXISTS trigger_ai_prompts_updated
+DROP TRIGGER IF EXISTS trigger_ai_prompts_updated ON ai_prompts;
+CREATE TRIGGER trigger_ai_prompts_updated
     BEFORE UPDATE ON ai_prompts
     FOR EACH ROW
     EXECUTE FUNCTION update_updated_timestamp();