Add data seeding and fix waitlist timestamp constraint

Introduces DataInitializer (ApplicationRunner) that populates the DB on startup with 4 platform stats, 6 features, 8 categorized FAQs, and 6 Moroccan real estate properties. All seeding is idempotent (count > 0 guard). Fixes a 500 on POST /api/landing/waitlist caused by a NOT NULL constraint on created_at — AbstractAuditingEntity has no JPA auditing listener in this service, so createdAt/updatedAt must be set explicitly on every entity save.