feat(01-03): register ACF field group for job_offer metadata

- Add DDHH_JM_ACF_Fields class with register_fields() method
- Field group 'Job Details' for post_type job_offer
- 6 fields: job_location (text, required), job_type (select, required),
  job_deadline (date_picker), job_contact_email (email, required),
  job_logo (image), job_deactivation_reason (textarea, conditional)
- German labels: Standort, Art, Bewerbungsfrist, Kontakt-E-Mail, Logo
- Logo returns ID for media library integration
- Deactivation reason field shows only when status != publish
- Hooked to 'acf/init' action

includes/class-ddhh-job-manager.php

@@ -49,5 +49,8 @@ class DDHH_JM_Job_Manager {
 
 		// Initialize post types
 		add_action( 'init', array( 'DDHH_JM_Post_Types', 'register' ) );
+
+		// Initialize ACF fields
+		add_action( 'acf/init', array( 'DDHH_JM_ACF_Fields', 'register_fields' ) );
 	}
 }